The Collaborative International Dictionary of English v.0.48:

Exogamy \Ex*og"a*my\, n.
   The custom, or tribal law, which prohibits marriage between
   members of the same tribe; marriage outside of the tribe; --
   opposed to endogamy. --Lubbock.
   [1913 Webster]

WordNet (r) 3.0 (2006):

exogamy
    n 1: marriage to a person belonging to a tribe or group other
         than your own as required by custom or law [syn: exogamy,
         intermarriage] [ant: endogamy, inmarriage,
         intermarriage]