Noun

1. The four-dimensional continuum of the three spatial dimensions plus time.

   An event is a point in spacetime, specified by the coordinates x,y,z and t.

2. An n-dimensional continuum consisting of dimensions of both space & time. Normally spacetime is considered as having 4 dimensions (x, y, z, t), but higher-dimensional spacetimes are often encountered in theoretical physics, e.g. the 5-dimensional spacetime of Kaluza-Klein theory or the 11 dimensions of spacetime in M-theory.
3. A specific region of the universe with mathematically different properties than the surrounding spacetime. Synonymous with "metric" within the context of general relativity.

   "a Schwarzschild spacetime," "a Reissner-Nordström spacetime," etc. as opposed to sense (2) describing the universe's spacetime as a whole: "a Minkowski spacetime," "a 5-dimensional spacetime," etc.