Dimension (from Latin "measured out") is, in essence, the number of degrees of freedom available for movement in a space.In common usage, the dimensions of an object are the measurements that define its shape and size. That usage is related to, but different from, what this article is about. Also, in science fiction, a "dimension" can also refer to an alternate universe or plane of existence, though this meaning is not discussed in this article.

** Physical dimensions **

The spacetime in which we live appears to be 4-dimensional. It is conventional (and for most practical purposes entirely sensible) to consider this as three spatial dimensions and one of time. We can move up-or-down, north-or-south, or east-or-west, and movement in any other direction can be expressed in terms of just these three. Moving down is the same as moving up a negative amount. Moving northwest is merely a combination of moving north and moving west.

Time is often referred to as the 'fourth dimension'. It is somewhat different to the three spatial dimensions in that there is only one of it, and movement seems to be possible in only one direction. On the macroscopic scale that we perceive, physical processes are not symmetric with respect to time. However, at the subatomic Planck scale, almost all physical processes are time symmetric (ie. the equations used to describe these processes are the same regardless of the direction of time), although this doesn't imply that subatomic particles can move backwards in time.

Theories such as string theory predict that the space we live in has in fact many more dimensions (frequently 10, 11 or 26), but that the universe measured along these additional dimensions is subatomic in size.

In the physical sciences and in engineering, the dimension of a physical quantity is the expression of the class of physical unit that such a quantity is measured against. The dimension of speed, for example, is length divided by time. In the SI system, the dimension is given by the seven exponents of the fundamental quantities. See Dimensional analysis.

** Mathematical dimensions **

In mathematics, no definition of dimension adequately captures the concept in all situations where we would like to make use of it. Consequently, mathematicians have devised numerous definitions of dimension for different types of spaces. All, however, are ultimately based on the concept of the dimension of Euclidean n-space E n.

A tesseract is an example of a four-dimensional object.

In the rest of this article we examine some of the more important mathematical definitions of dimension.

**Metaphysics**

Dimensions, as viewed psychically in out of body experiences - refers to moving one's conscious awareness through the grids that create our reality.

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