1.NounThe 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.NounAn n-dimensional continuum consisting of dimensions of both space and 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.
"a Minkowski spacetime; a 5-dimensional spacetime"
3.NounA 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"