Is weight a scalar or a vector? My professor insists that weight is a scalar. I sent him an email explaining why it's a vector, I even sent him a source from NASA clearly labeling weight as a vector. Every other source also identifies weight as a vector.
I said that weight is a force, with mass times the magnitude of gravitational acceleration as the scalar quantity and a downward direction. 
His response, "Weight has no direction, i.e., it is a scalar!!!" My thought process is that since weight is a force, and since force is a vector, weight has to be a vector. This is the basic transitive property of equality.
Am I and all of these other sources wrong about weight being a vector? Is weight sometimes a vector and sometimes a scalar?
After reading thoroughly through his lecture notes, I discovered his reasoning behind his claim: 

Similarly to how speed is the scalar quantity (or magnitude) of velocity, weight is the scalar quantity (or magnitude) of the gravitational force a celestial body exerts on mass. 

I'm still inclined to think of weight as a vector for convenience and to separate it from everyday language. However, like one of the comments stated, "Definitions serve us."
 A: I wouldn't get too hung up on this. The gravitational force exerted on an object is most definitely a vector, but on (the surface of) the earth, you don't need to make much distinction because for all intents and purposes, it always points in the same direction -- downward. 
I suspect the issue may be one of terminology. Yes, if "weight" refers to the force, it is a vector. But your teacher may very well be referring to the magnitude on the force, i.e. the number that you see if you put it on a spring scale. Again, on the surface of the earth, the magnitude conveys essentially the same information as the vector because the direction is known, and you typically don't use the word "weight" in other circumstances. 
EDIT: Also, I'm not sure but you might be confusing something. 

I said that weight is a force, with mass as the scalar quantity with a downward direction.

Mass, being a scalar, has no direction... did you mean to say something different?
A: It seems that your professor has confounded weight with mass.
Mass is a scalar, given in (kilo)gram as SI/CGS unit and never changes its value alone by changing the location. 1 kilogram is 1 kilogram on earth, in 10 000 km over earth, on the Jupiter or Sun or in a space station in weightlessness. 
Weight is, as you already mentioned, a vector and given in Newton/dyn as SI/CGS unit. It is the force a given mass is accelerated towards another mass. Therefore it changes by the location. If you have a rope, you give its strength in Newton (most prefer daN, dekanewton because it is almost the same force as 1 kg on normal earth). The same rope holding barely a mass on Earth will snap on Jupiter, but can hold six times the same mass on the moon.
So you are right. Yakk's answer is incorrect because the vector component is not neglible. If you make precise measurements, you will see that mountains or regions with high density changes the direction of the weight, you cannot say anymore it is pointing to the barycenter of the earth. Yes, physicists are sometimes also sloppy in everyday life and use mass/weight interchangably on earth or use magnitude of forces as a shortcut, but weight is a vector.
A: If weight is not a vector then why is it that there is a position between the Earth and the Moon where your weight is zero.
At this point the gravitational attraction on you due to the Moon (your weight due to the Moon) is equal in magnitude but opposite in direction to the gravitational attraction on you due to the Earth (your weight due to the Earth).
A: On earth, weight of a body is defined as the force by which the body is attracted by the earth towards its center. Weight can thus be considered the same as the gravitational force exerted by the earth on that body. Hence, weight can be deemed a vector since it is a force, irrespective of the planet you consider.
$$\vec W=m\vec g=\frac{GMm}{r^2}\hat r$$
As mentioned in the comments, since $g$ has the same direction (directed towards the center of the concerned planet) always, it might be(?) considered a scalar. Thats what your prof is doing. But strictly speaking, weight is a vector.
Hope this helps you.
A: As noted in many answers, weight is a vector.
The subtext of your question - how to deal with an instructor who makes mistakes - however, is harder to answer. Many people, myself included, don't like admitting their mistakes. I certainly wouldn't recommend, however, being confrontational, questioning their qualifications or work, or making remarks about their age. If possible, just understand the matter to your satisfaction, then let it go.
A: We can change the definition of things whenever it is useful.  Definitions serve us.  If a definition isn't useful, individuals and communities change it, sometimes on the fly, sometimes in context, sometimes explicitly, sometimes implicitly.
In everyday experience, weight is a scalar.  You don't write down the direction of the weight of the bananas you buy.  Insisting it is a vector is not useful in this context, and definitions exist to both clarify communication and solve problems.
Does adding the direction to the weight of the banana help solve any problems?  Or is it noise?  Is the scalar weight of the banana a communication problem in this case?
There are going to be other contexts where you will want weight to be a vector; maybe when calculating orbital mechanics of your banana.  Even there, weight may not be a useful concept, because there are much better ways to solve orbital mechanics than talking about the directional weight of things (field potentials, say).
In formal mathematics, very specific and exact definitions are used to permit abstractions that don't match any physical situation to be discussed and delt with in a uniform way.  Formal mathematics is often pillaged by physics, but physics isn't formal mathematics.
Physicists and Engineers will go off and talk about dirac delta functions whose value is 0 everywhere except at 0, and whose integral from any negative value to a positive one is 1, and then they convolve it with another function.
Now, there are ways to formalize this, but for the most part Physicists and Engineers don't bother.  "The Dirac Delta isn't a function" is useful when formalizing it, but isn't nearly as useful when working with it.  Knowing the formalization can be useful to avoid possible pitfalls, but it isn't usually useful when trying to use it as a tool to predict behaviour of some system.
Physics is a game of using mathematics (or whatever other tool is handy) to predict (and sometimes explain) the behavior of physical systems.  There are often multiple different mathematical games, and you will use different ones for different systems.  Newtonian dynamics is a game that works within its domain, and in it velocity is additive.  Relativity is a game that is overly complex for some domains, but covers some territory that Newtonian dynamics doesn't cover; in Relativity, velocity isn't additive.  Within Newtonian dynamics, velocity is a simple vector in a Euclidean space.  Within Relativity, it isn't a simple vector in a Euclidean space.
Weight is a scalar within some games of Physics.  In others, it may not be.  In almost every reasonable situation you will experience, Weight will be a Scalar, because in almost every game of Physics where the direction of Weight is important, using a vector-based Weight isn't going to be the best tool you have.
A: Mass is a scalar; weight is a vector.  Mass does not change regardless of gravitation field, but weight (to be precise) is the sum of vector components from all gravitation fields that attract an object.
For example, even on the surface of the Earth the Moon exerts some minute vector component that sums with the Earth's to give you the exact weight of an object, which will depend on the magnitude and direction of the vector component directed toward the Moon's center of mass, as well as on the component directed toward the Earth's center of mass.  This becomes even more significant if an object is in space somewhere between the Earth and the Moon.
To specify a precise weight, one must consider all the components of the weight vector.  The magnitude and the direction of a weight vector are dependent on its components.  If you want to weigh the oceans precisely, you need to specify their tidal positions.
In short, if a quantity is the sum of vectors, the quantity itself must be a vector.
However, practically speaking, it's impossible to solve a 3-body Kepler problem exactly.  So, in the absence of certainty about the position of each planet exerting gravitational pull on an object, it would be futile to attempt a precise sum of all the vector components contributing to an object's weight.  That (and the insignificance of the influence of other planets in the Solar System on an object at the Earth's surface) may be one reason that extra-terrestrial vector components of an object's weight usually are ignored.
In common usage "weight" is taken to mean only the vector pointing toward the Earth's center of mass.  That may be why some people use the word "weight" as though it were solely a magnitude, like a scalar, because the direction of the weight vector is tacitly assumed, and is left unstated, and insignificant other components of the weight vector are ignored.
A: Everything that we can resolve with some reference axis is a vector
As acceleration can be resolved in $x$,$y$ and $z$ direction is therefore it's a vector quantity.
Vector's have direction. 
Force also have direction.
Therefore weight is definitely a vector quantity.
A: I've always defined weight as the magnitude of the force exerted by gravity—the convention @knzhou and @gogators refer to in the comments. Wikipedia also mentions this convention, citing Halliday, Resnick, and Walker's Fundamentals of Physics (8th ed.) as an example of a textbook that defines weight in this way. (It may not be a coincidence my first few physics courses used this book.) I was surprised to learn that this convention isn't the most common one.
If you keep studying physics or math, you'll often run into situations where several conflicting definitions of a term coexist, even though you may think one of them is obviously better than the others. For example:


*

*In special relativity, some people define energy and momentum as the time and space components of the 4-momentum, respectively. Others define energy as the magnitude of the 4-momentum, and momentum as the 4-momentum itself.

*In differential geometry, some people allow manifolds to have boundaries. Others reserve the term manifold for manifolds without boundaries, using the term manifold with boundary when boundaries are allowed.

*(This isn't really a terminology conflict, but I can't resist mentioning it. When reading physics or math in French, beware of the false cognate positif. It sounds like it means positive, but it actually means nonnegative! The French term for positive is strictement positif.)
These conflicts of convention don't cause any problems, as long as everyone is aware that multiple conventions exist, and everyone is careful to say which convention they're using. They probably aren't going away anytime soon, so I recommend getting used to them.
A: If he is a PhD and you are a Major in physics there is no reason to take his word above your own in this case. Physicists don't get a rerun of "basic concepts in mechanics" after majoring. Most likely he knew as much about weight when he was a major as he does now. And you will also not get further education on these basic concepts.
However, if instead of arguing about misuse of terms you want to try to understand, ask him "how do we define weight in the context of this course?" For example, it might be that in the particular field it is handy to use the weight's component/projection that is normal to the surface on which the object is. In that case it would be normal to have scalar weight and call it "weight" because of tradition or handiness reasons.
But there is always a chance that your professor is just not very insightful and was never interested to find out and correct his misconceptions of basic subjects. PhD is earned in a subfield, it doesn't automatically make you bright.
A: For any vector physical quantity it may make perfect sense to define a corresponding scalar quantity equal to that vector's magnitude. Unsurprisingly, both those quantities will usually share a common name.
For example, acceleration is a vector, but 9.8 m/s² is certainly a scalar value, there's no notion of direction in it whatsoever.
Of course, you may call the latter acceleration magnitude if you want to be pedantic, but people who are calling it just acceleration (and asserting it's a scalar) are not outright wrong.
A: Weight is a force, so it is a vector. 
A: As everyone else has said, this debate is mostly a definitional quibble about exactly how the word "weight" is defined.  Two thoughts:
(a) Many people have pointed out that in colloquial speech, the word "weight" usually does not include a direction.  But in colloquial speech there almost no words that are commonly used to indicate both a magnitude and a direction.  E.g. when people colloquially use the word "velocity," they're almost always only referring to a magnitude, not to a direction.  That's why the concept of vectors needs to be taught to beginning students - it's not totally intuive. So if you're going by the colloquial rather than the scientific definition of words, then arguably nothing counts a vector.
(b) In the discussion of aerodynamics in particular, there are approximately ten gajillion diagrams that look like this:

It would be hard to make sense of this diagram if you were considering weight to be a scalar.
A: I think we just can follow Wikipedia definition:

In science and engineering, the weight of an object is usually taken
  to be the force on the object due to gravity. Weight is a vector whose
  magnitude (a scalar quantity), often denoted by an italic letter W, is
  the product of the mass m of the object and the magnitude of the local
  gravitational acceleration g.

A: In colloquial language, weight is often set equal to mass. For example: "My weight is 70 kilogrammes." No one says: "I weigh 700 Newton" [assuming $g=10(\frac {m} {sec^2})$]. All weight scales show you a number. Gauged for the earth (and standing on the scale on earth) this is your mass. But on the moon, the weight scale shows you a different number. This means that what the number the scale shows you is not your mass (except on earth) but a measure of the force that's acting on you. So what you actually measure with a scale is the force. So weight is a force, but only on earth, you can see directly your mass while standing on a scale. Gauged for the moon, off course you would see the same mass, but multiplied by the moon's gravitational acceleration (which is a vector, and thus weight is one too) you'll weigh less than on earth. Putting an earth gauged scale on the moon, you'll see a smaller number and people are right by saying that you weigh less on the moon, but because weight is confused with mass, many also think their mass is less on the moon. 
A: If "weight" is understood as a force of gravity, then it is a vector, because force is a vector. Mass is the scalar value that can be used to compute the gravity force.
When gravity force is computed (F = mg), scalar (m) is multiplied to vector (g), making the result a vector.
A: The weight $\small\begin{pmatrix} 0 \\ 0 \\-87\text{ N}\end{pmatrix}$ is a vector in 3D space,
whereas the weight $-87$N is a scalar (and a vector in 1D space).
On the other hand, if weight's direction (towards the centre of Earth) is considered to be a given, then it makes sense to specify weight just as a magnitude (i.e., a nonnegative scalar) like $87$N.
A: I think your professor is mixing up terms.  Mass is scalar, weight is a vector.  But many people get into the habit of using the terms interchangeably.  Also, don't always believe everything someone in a "superior position" tells you.  Sometimes they are wrong, so question everything.
A: According to Wikipedia,
"In science and engineering, the weight of an object is related to the amount of force acting on the object, either due to gravity or to a reaction force that holds it in place.
Some standard textbooks define weight as a vector quantity, the gravitational force acting on the object. Others define weight as a scalar quantity, the magnitude of the gravitational force. Others define it as the magnitude of the reaction force exerted on a body by mechanisms that keep it in place: the weight is the quantity that is measured by, for example, a spring scale. Thus, in a state of free fall, the weight would be zero. In this sense of weight, terrestrial objects can be weightless: ignoring air resistance, the famous apple falling from the tree, on its way to meet the ground near Isaac Newton, would be weightless.
The unit of measurement for weight is that of force, which in the International System of Units (SI) is the newton. For example, an object with a mass of one kilogram has a weight of about 9.8 newtons on the surface of the Earth, and about one-sixth as much on the Moon. Although weight and mass are scientifically distinct quantities, the terms are often confused with each other in everyday use (i.e. comparing and converting force weight in pounds to mass in kilograms and vice versa).
Further complications in elucidating the various concepts of weight have to do with the theory of relativity according to which gravity is modelled as a consequence of the curvature of spacetime. In the teaching community, a considerable debate has existed for over half a century on how to define weight for their students. The current situation is that a multiple set of concepts co-exist and find use in their various contexts."
Therefore, weight is a force.
If your physics professor is still refusing to listen, then **Tell your physics professor to check out these websites:


*

*https://en.wikipedia.org/wiki/Weight

*https://www.thestudentroom.co.uk/showthread.php?t=864241

*https://www.quora.com/How-is-it-that-mass-is-a-scalar-quantity-but-weight-is-a-vector-quantity

*https://socratic.org/questions/which-of-the-following-are-vectors-and-which-are-scalars-distance-mass-time-weig
A: Here, I would suggest using NASA as more authoritative than you teacher to eliminate opinion.  https://www.grc.nasa.gov/www/k-12/airplane/vectors.html
Directly from their opening summary:  Scalars were quantities without direction, including length, speed, volume, area, mass, density, pressure, temperature...
Vectors are quantities with direction: displacement, velocity, acceleration, momentum, force, lift, drag, thrust, weight.
I have never been sure why, but for some reason weight and mass are one that is often flipped with people incorrectly claiming that mass is a vector and weight is not and not understanding that weight does have direction: towards the center of force causing it, in our case gravity so down.
