ETYM Old Fren. afinité, French affinité, Latin affinites, from affinis. Related to Affined.
Close relationship; attraction; resemblance; Law, relationship by marriage; Chemistry, force causing elements to combine.
1. A natural attraction or feeling of kinship:.
2. Inherent resemblance between persons or things.
3. A close connection marked by community of interests or similarity in nature or character; SYN. kinship.
4. The force attracting atoms to each other and binding them together in a molecule; SYN. chemical attraction.
5. (Immunology) The attraction between an antigen and an antibody.
6. (Biology) State of relationship between organisms or groups of organisms resulting in resemblance in structure or structural parts: SYN. phylogenetic relation.

In law, relationship by marriage not blood (for example, between a husband and his wife's blood relatives, between a wife and her husband's blood relatives, or step-parent and stepchild), which may legally preclude their marriage. It is distinguished from consanguinity or blood relationship.

In chemistry, the force of attraction (see bond) between atoms that helps to keep them in combination in a molecule. The term is also applied to attraction between molecules, such as those of biochemical significance (for example, between enzymes and substrate molecules). This is the basis for affinity chromatography, by which biologically important compounds are separated.
The atoms of a given element may have a greater affinity for the atoms of one element than for another (for example, hydrogen has a great affinity for chlorine, with which it easily and rapidly combines to form hydrochloric acid, but has little or no affinity for argon).
The affinity of monoclonal antibodies for specific proteins forms the basis of many medical diagnostic tests, for example pregnancy testing.

For Network Load Balancing, the method used to associate client requests to cluster hosts. When no affinity is specified, all network requests are load balanced across the cluster without respect to their source. Affinity is implemented by directing all client requests from the same IP address to the same cluster host. See also client request, IP address.