In general, the fourth dimension is the dimension we live in, containing
the dimensions length, height, width and time. This dimension is rarely
spoken of, because most people don't know time is dimension as well.
In mathematics the fourth dimension, or a four-dimensional ("4-D") space, is an abstract concept derived by generalizing the rules of three-dimensional (3-D) space. It has been studied by mathematicians and philosophers for almost two hundred years, both for its own interest and for the insights it offered into mathematics and related fields.