A dimension is a variable in which the position of an event can vary. We know of four dimensions in our universe: three of which are present in space, and the fourth of which is time.

String theory predicts that there are more dimensions that particles can move around in but which are difficult to observe because of the mathematically complicated physics that string theory describes. None of these other dimensions have remotely compelling evidence to support them, however, so most scientists prefer to use simpler theories that use only the four dimensions that we know about.