An ironic term coined in 1950 (during a BBC programme) by the British astrophysicist Fred Hoyle to refer to the "original explosion" concept introduced into the cosmological model initially developed in the 1920s by the Belgian astrophysicist Georges Lemaître and the Russian physicist Alexander Friedmann.
In the Friedmann and Lemaître model, the universe seems not to have existed eternally in the past and there did exist an initial singularity which is often awkwardly described as a sort of explosion, which inspired Hoyle. But the important physical prediction of this first version of the model is an expansion phase of the universe, an idea which was upheld from 1929 by the observations of the American Edward Hubble. In the 1940s, the precise physical description of this expansion phase really began, under the impulsion of the Russian born American physicist George Gamow. He theoretically predicted the existence of a primordial phase during which the universe was an extremely dense, hot and opaque object (predictions upheld in 1965 by the accidental observation of the cosmic microwave background by the Americans Arno Penzias and Robert Wilson). It was to provocatively describe this cosmological model, a competitor of his own (the so-called "steady-state universe") that the term Big Bang was introduced by Hoyle.
While the term Big Bang initially referred only to "time 0", it is now often used to describe the standard cosmological model in its entirety. Thus, the expression "standard model of the Big Bang" refers to a cosmological model in perpetual evolution, but based on the Friedmann-Lemaître-Gamow model (to whom are sometimes added the names of the Americans Howard Robertson and Arthur Walker) and incorporating various other elements, such as an "inflation" phase.