Asymptotically flat spacetime is a concept in general relativity that describes the behavior of spacetime in regions that are far away from any gravitational sources, such as stars or black holes. In this context, "asymptotically flat" refers to the idea that as one moves far from the influence of mass and energy, the geometry of spacetime approaches that of flat Minkowski space, which is the simplest model of spacetime in special relativity.
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An asymptotically flat spacetime approaches Minkowski spacetime sufficiently far from isolated sources. At null infinity this is stated invariantly by requiring a smooth conformal completion whose boundary contains future and past null infinity.