WordNet (r) 3.0 (2006):

phase space
    n 1: (physics) an ideal space in which the coordinate dimensions
         represent the variables that are required to describe a
         system or substance; "a multidimensional phase space"

The Free On-line Dictionary of Computing (30 December 2018):

phase space

    The 2n-dimensional space consisting of the
   possible values of (x1, x1., x2, x2., ..., xn, xn.) for a
   system of n first-order ordinary differential equations (or
   more generally, Pfaffian forms).

   If n=1, the phase space is known as a phase plane.

   (http://mathworld.wolfram.com/PhaseSpace.html).

   (2008-05-02)