The Collaborative International Dictionary of English v.0.48:

space-time \space-time\, n.
   the four-dimensional coordinate system in which all physical
   objects of the known universe are located, and in which all
   physical events occur; it consists of three spatial
   dimensions and one time dimension; -- also called the
   space-time continuum.
   [PJC]

WordNet (r) 3.0 (2006):

space-time
    n 1: the four-dimensional coordinate system (3 dimensions of
         space and 1 of time) in which physical events are located
         [syn: space-time, space-time continuum]