The Collaborative International Dictionary of English v.0.48:

Melting pot \Melt"ing pot\ n.
   1. A vessel in which anything is melted; a crucible.
      [1913 Webster]

   2. (Sociology) (fig.) A place where people of different
      backgrounds become similar in culture. The United States
      has often been referred to as a melting pot, though the
      differences in cultures of recently arrived immigrants
      persists beyond the generation of immigrants.
      [PJC]

WordNet (r) 3.0 (2006):

melting pot
    n 1: an environment in which many ideas and races are socially
         assimilated
    2: a vessel made of material that does not melt easily; used for
       high temperature chemical reactions [syn: crucible,
       melting pot]