Great question. In fact it is a very difficult question to answer as there is no right answer at all. Time a a measurement of course, but it is also a philosophical issue and a concept from physics.
From a physics point of view, there is an underlaying process of motion and forces from which time emerges, however what we perceive as time is mostly an illusion. Our memory creates the illusion of the past. Conscious perception of events gives the feeling of present. Future is a mental structure based on memory experience of the past. Concept of time emerges as our mind tries to make sense of the world around us which is filled with continuous change.
A great link is found here which tries to explain this again.
That is too hard! How about “a parameters that determines causality”. This isn’t very satisfying – But it does hint at an interesting property of time – an event can only cause something to happen at later times. So, you reading this cannot cause me to type it! This is different to be motivated to write this in the hope that someone might read it.
Difficult question – difficult answer 😉
Something else that always increases with time is a property called “entroy” which is a formal way of measuring disorder. So, the entropy of the universe always increases, which means that it is becoming increasingly disordered – much like my desk at work … ! This gives time a clear direction or orientation, which is not true of space.