Big Bang Definition

1. A hypothetical model of the origin of the universe, now generally accepted, which postulates that all matter and energy were once concentrated into an unimaginably dense state, which underwent a gigantic explosion between 13 and 20 billion years ago.
(Brit)
2. colloq
The introduction, on 27 October 1986, of a momentous package of changes to the rules controlling the British Stock Exchange (including the abolition of the distinction between jobbers and brokers).