The Big Bang describes the origin of the universe from a single hot, primeval fireball and the term was originally a pejorative label created by proponents of the steady-state theory.

“The Big Bang itself, it was an equal competitor to the steady-state theory for decades, until we finally got some evidence to support the Big Bang, and that was the famous cosmic microwave background. This is a leftover signal, signature, from an explosion, that started in one hot, primeval fireball”