The Collaborative International Dictionary of English v.0.48:

big bang \big bang\ n. (Astron.)
   The explosive event marking the beginning of the known
   universe, according to big bang theory; the beginning of
   time.

   Note: The big bang is likened to a massive explosion from a
         point containing all of the matter and energy of the
         beginning universe, which started the expansion and
         evolution of the universe which continues today. The
         evidence for such an event includes the apparently
         expanding size of the present universe, and a
         background microwave radiation of about 3[deg] Kelvin,
         equal to that predicted from mathematical models of
         such an explosive event.
         [PJC]

WordNet (r) 3.0 (2006):

big bang
    n 1: (cosmology) the cosmic explosion that is hypothesized to
         have marked the origin of the universe