The Penrose–Hawking singularity theorems are a set of results in general relativity which attempt to answer the question of when gravitation produces singularities.
A singularity in solutions of the Einstein field equations is one of two things:
a situation where matter is forced to be compressed to a point (a space-like singularity)
a situation where certain light rays come from a region with infinite curvature (a time-like singularity)