Irregular boundary of spacetime in general relativity – region where spacetime simply comes to an end. Often, such boundaries are associated with spacetime curvature growing beyond all bounds and becoming infinitely large – so-called curvature singularities (notably Ricci singularities or Weyl singularities) – but there are exceptions (for instance a conic singularities).
According to general relativity, there exists a singularity inside every black hole, and the starting point of any universe described by a big bang model is a singularity, as well. The occurence of sinularities is a failure of general relativity – and a strong indication that the theory is incomplete. Instead, one such describe the earliest universe and the interior of black holes using a theory of quantum gravity.