Time travel is the concept of moving between different moments in time in a manner analogous to moving between different points in space, either sending objects (or in some cases just information) backwards in time to a moment before the present, or sending objects forward from the present to the future without the need to experience the intervening period (at least not at the normal rate). Time travel has not been proven to be possible. Any technological device, whether fictional or hypothetical, that is used to achieve time travel is known as a time machine. ( Edited by plmiller )  

http://en.wikipedia.org/wiki/Time_travel