Short definition:A Black Hole is a region of spacetime where the gravitational field is so strong that nothing, not even light or other electromagnetic waves can escape its event horizon. An event horizon is “the point of no return”, meaning the boundary beyond which nothing can escape.
Detailed definition: Since Black Holes can’t be observed directly with telescopes, they’re usually detected by other means, such as observing their gravitational influence on their surroundings.
Most Black Holes are formed from large stars that die in a supernova explosion - these Black Holes are usually around 20 times as massive as the Sun. However, there also exist Black Holes that are incredibly large, called Supermassive Black Holes, which can be millions or even billions times as massive as the Sun. Scientists believe that at the centre of almost every big galaxy lies a Supermassive Black Hole, for example Sagittarius A* at the centre of the Milky Way.
Etymology: Presumably in December 1967, a student suggested the phrase "black hole" at a lecture by John Wheeler; Wheeler adopted the term for its brevity and "advertising value", and it quickly caught on. (Source: Siegfried, T. (2019, August 9). 50 years later, it’s hard to say who named Black Holes. Science News. https://www.sciencenews.org/blog/context/50-years-later-its-hard-say-who-named-black-holes)
