1. A theory of the origin of the universe in which all matter and space originated in a cataclysmic explosion, the remnant of which we see in the present expansion of the universe as a whole.
2. The hypothetical "massive explosive" event that is said to have marked the origin of the universe as we know it, about 18 billion years ago.

According to the theory, at the time of the Big Bang, the entire universe was squeezed into a hot, superdense state and the explosion threw the compact material outward, producing the expanding universe.