[Omni]

Let's say you're somewhere in the universe and you moving is not made impossible by obstruction of some sort then you can move in every direction. It's called 3d and movement implies time so why not call time also a dimension and there you have it...4d. 3d means where you have length, width and height at the same time and you can turn an object all ways possible and it will always have length, width and height at the same time. I gues that pretty much sums it up besides ofcourse there are people thinking that 1d, 2d, 9d and "fill in your own favorite-d" are possible.