Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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