What condition are to be fulfilled for a circuit to be short-circuited? So I am curious about how short-circuiting works. If I google, I can find some general concepts like the current is too high. But what makes a current too high? So for example if I had a battery of 5 volts with an internal resistance of 1 ohm, would there be some value for an external resistor which determines whether the circuit is short-circuited?
 A: I've seen a few definition.  I'll go from the simplest to the most complex.  Most likely the simple ones will suffice.
The most common definition of a short circuit is any time the equations for the model of your circuit suggest that a single wire must have two different voltages.  As you know an ideal wire has the same potential at all points along the wire.  If your equations suggest that a wire must be held at 2 different voltages at once, you have a short circuit.
The most obvious example would be a simple loop with a wire going from the postive side of a 5V battery to the negative side.  For convenience, let's take a reference voltage (ground) at the negative side of the battery.  The wire is connected to the negative side, so its voltage must also be 0V.  However, because the wire is also attached to the positive side, the voltage on the wire must be 5V.  The wire cannot be both -- this is a short circuit.
What happens in these cases is that your ideal model breaks down, and the actual physical behavior depends on a lot of non-ideal behaviors which you usually get to ignore.  For example, the battery has an "internal" resistance, and the wire has a resistance as well.  When you consider these, you'll see that the wire will actually act more like a long resistor, rather than a perfect ideal wire.
This brings us to a second definition, which you won't see until later.  If you get into circuit design, you might end up modeling circuits at a fidelity to where you actually can model what happens when you connect two wires you shouldn't have.  In these cases, the simpler definition of "a wire at two voltages" breaks down because you built your model to include these effects.  In these cases, a short circuit gets defined as any low-resistance path which causes the circuit to behave outside of its design specs.  As an example, in Radio Frequency circuits, you'll find a lot of frequency dependent behaviors.  You have to model these carefully.  You may find a resonant frequency of your circuit which puts so much energy into one part of your circuit that you simply didn't plan on it, even though you had the models to do so.
Finally, there's a non-terminal version.  Sometimes we'll use the word "short" to describe a very intentional low-resistance path.  There's many cases in electrical engineering where you may be told to "short a wire to ground," and doing so was in the design specs.  One example is in busses with pull-up resistors.  A very common bus design is to have a communication wire going between two chips that has a "pull up" resistor connecting it to power (5V).  This construct ensures the wire is at 5V, which both chips see.  If you "short it to ground," that means that you connect the wire directly to ground.
If you see such a scenario, you'll realize that there is no "short circuit" as described in the first two definitions.  You just have a connection from 5V through a resistor to ground, and the entire bus wire is held at 0V.  There's nothing spectacular about this connection at all.  However, we will call it a "short circuit" anyways.  Why?  Well, language works in mysterious ways.  However, if I had to venture a guess, I'd say it's because connecting something to ground that wasn't already connected that way is one of those actions that an electrical engineer pays attention to.  We've created short circuits that way before.  In this case, that connection is intended.  By calling it a "short," the person writing the instructions makes it clear to the worker that, yes indeed, this is going to be a path to ground, and yes, they understand that such a connection would be bad if considerations weren't taken.
