1. A theory of the evolution of the universe that states that the universe has always been in the state it is now, which leads to the implication that the universe has no origin, but has always existed.
2. A rival theory to that of the Big Bang which claims that the universe has no origin but is expanding because new matter is being created continuously throughout the universe.

The theory was proposed in 1948 by Austrian-born British cosmologist Hermann Bondi, Austrian-born U.S. astronomer and physicist Thomas Gold, and English astronomer Fred Hoyle, but this concept was challenged in 1965 by the discovery of cosmic background radiation and is now largely rejected by scientists.