[AUgrad]

The generally accepted answer is that the fourth dimension is time. The more interesting question is "what is the fifth dimension?" (And no smart remarks about the musical group please)

You're getting over my head. I don't understand how time could be a dimension in this sense.

[lafroste]

If you look at dimensions as if they were coordinate axes in a geometry problem, then you already can see that the location of any point can be uniquely described by its coordinates, (x,y,z). However, a fourth coordinate can be employed to say when it was there. If the object is moving, then its (x,y,z) changes with time, and its position can be fully described by knowing (x,y,z,t).