What is the definition of a black hole?

A black hole is defined as a region of space-time, which has an intense gravitational field such that no matter or radiation can escape.

A black hole is a place where matter has collapsed in on itself. It is the final stage of a massive star which has undergone a supernova explosion.

This catastrophic collapse results in a huge amount of mass being squeezed in an incredibly tiny volume. The gravitational effect of this place is so great that nothing - not even any particle or light - can escape from inside it.

Because no light can get out, black holes are invisible.