What is time, with respect to universe? I want to know what is actually time, not like the measurable quantity. But with some scientific terms
Someone said that "time is entropy". Is that true?
also what makes time unique?
 A: In classical mechanics, time is the unique parametrisation of dynamical systems.
In relativity theory, one then sees that time is somewhat more than this, because there exists a global symmetry (the Poincaré-group) that involes time and space on an equal basis (called spacetime).
Also, one can show that parametrisation by time is not the only way to do physics right.
Sometimes one comes across the problem that a certain problem allows two different solutions called "advanced" or "retarded", meaning that the first doesn't encompass relativity-causality while the later does.
Performing a time reflection (t -> -t), the two solutions switch their behaviour in terms of causality but with the time flowing backwards.
This gives rise to the question of the "direction" of time flow and how to set it right.
One can now try to parametrisate time by entropy $t(S)$ (time rises with entropy and vice versa) because entropy seems to not have this problem of being "advanced" or "retarded".
In terms of modern quantum field theory, there exists a theorem which is called CPT-theorem. It states that performing a symmetry-transformation consisting of Charge-conjugation, Parity (x,y,z -> -x,-y,-z) and Time-reversal (t -> -t), the underlying physics do not change.
Experimentally we already know that this does not hold for CP-transformations alone.
Therefore one concludes that time-reversal is not a "good" transformation for physics, setting the direction of time flow in a unique way.
In my opinion, time flow is not a good term anyway. If one tries to think of the world on the basis of any relativty-theory, one should stick with "events" and rather leave "time-flow" aside.
A: "Time" is very difficult to define properly, in General Relativity time is the 4th dimension of what is known as "spacetime" and this dimension is one which you may only move "forward" and you must move through spacetime at the speed of light, which is why time slows down as you move near the speed of light.
Other than this, time is not well defined; you can even argue that time doesn't really exist and our minds arbitrarily choose to interpret time as the direction in which entropy increases and form our concept of causation around that progression.
A: I personally like the view pesented in this paper. So, different snapshots of the universe at dfferent times, should actually be considered to be different universes which are related to each other by being correlated. While awkward from an intuitive point of view, it is a far more natural from the point fo view of quantum mechanics. 
