Definition for Exogamy
From Biology Forums Dictionary
The social rule that one must marry someone from outside one's own group is called exogamy. Exogamy is a rule that requires an individual to marry outside of a culturally defined group of which he or she is a member.
All societies prohibit marriages between individuals of the same group because they consider that members of the same group are brothers and sisters. For this reason they stipulate that an individual must seek a wife not from his own group but from another group. This especially there with regard to descent groups. Each descent group is a collectivity of people who arose from a common ancestor. Thus exogamy favors marriage outside one's own group. It is culture which decides the exact limits of one's own group.