The word affinity (Lat. affinitas, relationship by marriage, from affinis, bordering on, related to; finis, border, boundary) bears several meanings.

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.

The subject is chiefly important from the matrimonial prohibitions by which the canon law has restricted relations by affinity. Taking the table of degrees within which marriage is prohibited on account of consanguinity, the rule has been thus extended to affinity, so that wherever relationship to a man himself would be a bar to marriage, relationship to his deceased wife will be the same bar, and vice versa on the husband's decease. See affinity (canon law) for more discussion.

Briefly, direct affinity is a bar to marriage. This rule has been founded chiefly on interpretations of the eighteenth chapter of Leviticus. Formerly by law in England, marriages within the degrees of affinity were not absolutely null, but they were liable to be annulled by ecclesiastical process during the lives of both parties; in other words, the incapacity was only a canonical, not a civil, disability. By the Marriage Act 1835 all marriages of this kind not disputed before the passing of the act were declared absolutely valid, while all subsequent to it were declared null. This rendered null in England, and not merely voidable, a marriage with a deceased wife's sister or niece. (See consanguinity, marriage.)

Life sciences

Biology

In biology, affinity is a relationship between species that may share a common origin.

Immunology

In immunology, affinity is the selective attraction between an antibody and an antigen.

Mathematics

Occasionally, mathematicians may use the word affinity to mean the property of being affine.

Chemistry

In chemistry the term affinity is used with the meaning of reactivity, referring to the relative stability of the product(s) from a certain reaction or process. It is also seen as an attractive force between two particles. An example is electron affinity. Affinity of one chemical to another is measured quantitatively by an equilibrium constant of the bound and nonbound form.

Engineering

In engineering, affinity laws describe the relationship between turbomachinery's geometry and speed with performance, and is a subtopic of similitude.