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A phase space is a space of all possible states of a system, and for mechanical systems the phase space typically considered is a position and a momentum component for each dimension, i.e., six dimensions for 3-dimensional space. A system with one moving object has these six dimensions, and system with more than one moving object has these six dimensions for each such un-fixed object. (This treats them as point masses; for finite solid objects, there is more complication.) The concept is used in statistical mechanics and quantum mechanics.
Four dimensions are sufficient for an object traveling within a plane and two dimensions are sufficient for the phase space of a pendulum allowed to swing along a single path, one for its position along the path and another for its momentum. Such a two-dimensional system is termed a phase plane, and the term phase line similarly indicates just a single dimension of states.
A phase integral is an integral calculating a volume in a phase space. For some calculations, such a volume is useful as a substitution for discrete numbers of potential points within the phase space.