There are many definitions of time travel but most compare travelling through time with travelling in space. I suppose this makes sense in a way, although there is a definite difference: we have control when we travel through space, but we all travel through time by doing nothing, and we can do nothing about it. I’d argue that time travel, then, needs some sort of control in how we move through time.
I’ll write later on the speed of light and its importance in time travel, but the two must not be confused; for now I’ll clarify with an example:
When you look up at the stars, it is commonly said that you are “looking into the past”. This is because the stars are so distant that the light travelling from them takes several years to reach us…so we see them as they were this many years ago. But is this a form of time travel? Are we really looking into the past?
I don’t think so. I think this is more similar to watching a movie of say, your children, which was taken some time ago where again, we are watching past events. I suppose the difference is that you can watch those home movies again and again, whereas we can only see the stars for that one ‘real time’ moment.
I believe that a certain level of interaction is key for time travel. If I went back in time, I could qualify that statement by describing the people and places and things that I saw that I otherwise would not have been able to see. There is a new exchange of information. With a home movie, we can’t look out of the camera view, or talk to the people on film (or hear back from them). A time traveller, on the other hand, would be able to look wherever they wanted, or interact with the people and objects in the new time.
I hope that this has answered the question of “what is time travel?”
Further examples of what is and isn’t time travel can be found here.