What is the definition of "black hole"?

A black hole as an invisible spot in space where gravity is so intense that nothing can escape it. Even light is trapped by this celestial body. Black holes can form when a star has died and collapsed in on itself, becoming incredibly dense.

Black holes are hard to detect, but special telescopes in space are able to detect them by studying the movement of stars and gas around them. Scientists believe the Milky Way galaxy may be home to numerous black holes, and the size of black holes can range from the size of an atom to more than 1 million times the size of the sun.