Dimensions

Our world takes place in three dimensions of space: length, width, and height. Some may argue that we also have a fourth dimension, time, but time is not usually considered to be a dimension of movement/space.

Worlds
Edwin A. Abbott imagined multiple overlapping worlds, each consisting of a separate dimension. This idea was chronicled in his book Flatland: A Romance of Many Dimensions. For example, his one-dimensional universe ("Lineland"), where everybody is a line segment and can only travel in a straight line, is part of the 2-D world ("Flatland"), where each resident is a geometric shape. The final world is "Spaceland", a three-dimensional world (much like ours) where each Spacelander is a 3-D figure.

Each person in each dimension can only imagine their dimensions. For example, a Flatlander can only think of two directions (forward/back and right/left). They would not be able to comprehend any direction which we call "up" and "down" (the z-axis).