Four-dimensional space ( 4D ) is the mathematical extension of the
concept of three-dimensional space (3D). Three-dimensional space is the
simplest possible abstraction of the observation that one needs only three
numbers, called dimensions , to describe the sizes or locations of objects
in the everyday world. For example, the volume of a rectangular box is found
by measuring and multiplying its length, width, and height (often labeled x,
y, and z). This concept of ordinary space is called Euclidean space because it
corresponds to Euclid's geometry, which was originally abstracted from the
spatial experiences of everyday life.
The idea of adding a fourth dimension appears in Jean le Rond d'Alembert's
"Dimensions".