What's a black hole?

A black hole is called so because even light cannot pass through.

Explanation:

We can see things only when the light is reflected back and received by our eyes and further transmitted to brain by neurons. When when the substance is very dense and no light is reflected back one cannot see anything and then its called Black Hole. we perceive that there must be something but we cannot see because the light is not reflected back. Essentially the matter is so solid that nothing-no particles or even electromagnetic radiation, for example, light-can escape from it. The hypothesis of general relativity predicts that an adequately conservative mass can twist space time to shape a dark opening

Answer: A black hole is a star's core collapsed into extremely dense matter.