Law
In law and in cultural anthropology, affinity, as distinguished from consanguinity, is kinship by marriage. It is the relation which each party to a marriage, the husband and wife, bears to the kindred of the other. Affinity is usually described as of three kinds. (1) Direct: that relationship which subsists between the husband and his wife's relations by blood or between the wife and the husband's relations by blood. The marriage having made them one person, the blood relations of each are held as related by affinity in the same degree to the one spouse as by consanguinity to the other. But the relation is only with the married parties themselves, and does not bring those in affinity with them in affinity with each other; so a wife's sister has no affinity to her husband's brother. This is secondary affinity. Collateral affinity is the relationship subsisting between the husband and the relations of his wife's relations.

http://www.answers.com/topic/affinity-4