A better definition would be that we PERCEIVE time to be as a linear succession of events in OUR UNIVERSE. Because the arrow of time is caused by entropy the second law of thermodynamics. Which says that in an isolated system, entropy tends to increase with time. That's exactly what's been happening since the big bang.
I'm a big supporter of Albert's Einstein theory of relativity because all evidence we come up with to this day proves he was right. Time is four dimensional space (=spacetime).
https://en.wikipedia.org/wiki/Spacetime
We can only see things in 3d but there are more dimensions. Our multiverse can have up to 11 dimensions. What we call time is the 4th dimension.
Can you manipulate time?
There are 2 types of time travelling, travel to the future and travel to the past.
Travel to the future has actually been done a long time ago, Einstein showed us how. The first time traveller ever was "Sergei Avdeyev" a Russian cosmonaut.
https://en.wikipedia.org/wiki/Sergei_Avdeyev
What happened is he spend 747 days aboard Mir, cumulative across three missions, he went approximately 27,360 km/h and thus aged roughly 0.02 seconds (20 milliseconds) less than an Earthbound person would have. That's because of the effect of time dilation.
https://en.wikipedia.org/wiki/Time_dilation
"According to the theory of relativity, time dilation is a difference in the elapsed time measured by two observers, either due to a velocity difference relative to each other, or by being differently situated relative to a gravitational field. As a result of the nature of spacetime, a clock that is moving relative to an observer will be measured to tick slower than a clock that is at rest in the observer's own frame of reference. A clock that is under the influence of a stronger gravitational field than an observer's will also be measured to tick slower than the observer's own clock."
The problem with travelling to the future is that you need to fly as fast as possible. Ideally as fast as the speed of light but we just don't have rockets that are capable of doing such thing.
Travelling to the past is much more difficult. We don't even know if it's really possible. You would need a wormhole to travel to the past. Wormholes are a bridge between one location in space to another.
https://en.wikipedia.org/wiki/Wormhole
