Lensing by a black hole. Animated simulation of gravitational lensing caused by a Schwarzschild black hole going past a background galaxy.
A gravitational singularity (sometimes called a spacetime singularity) is a term used to describe the center of a black hole where gravity is thought to approach infinity.
In the center of each black hole is a singularity, a point where infinite density develops as spacetime approaches it. Spacetime goes toward infinite curvature and matter is crushed to infinite density under the pull of infinite gravity. At a singularity, space and time cease to exist as we know them and current laws of physics cannot be applied to this region.
Images for kids

A simple illustration of a nonspinning Black hole and its singularity