The super-dense particle at the core of a black hole, or the location where the quantities that are used to measure the gravitational field become infinite in a way that does not depend on the coordinate system. Singularities can be divided according to whether they are covered by an event horizon or not (naked singularities). According to general relativity, the initial state of the universe, at the beginning of the Big Bang, was a singularity. (From The Emergent Christ)