affinity

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A natural attraction, liking, or feeling of kinship.
• noun A natural tendency or ability to use or do something.
• noun A natural compatibility of one thing with another.
• noun Relationship by marriage.
• noun An inherent similarity between persons or things.
• noun Biology A relationship or resemblance in structure between species that suggests a common origin.
• noun An attraction or force between particles or chemicals that causes them to combine.
• noun The degree to which particles or chemicals are likely to combine.

from The Century Dictionary.

• noun In projective geometry, a perspective of which the center is at an infinite distance.
• noun An artificial relationship between persons of different blood, regarded as analogous to consanguinity; the relation between families or individuals created by intermarriage (excluding that between the married persons), by legal adoption, or by sponsorship; more especially, the relation between a husband or wife and the kindred of the other spouse.
• noun Intercourse; acquaintance; companionship.
• noun Hence A natural liking for, or attraction to, a person or thing; a natural drawing or inclination; an inherent mutual liking or attraction.
• noun Inherent likeness or agreement as between things; essential or specific conformity; intimate resemblance or connection.
• noun In chem., that force by which the atoms of bodies of dissimilar nature unite in certain definite proportions to form a compound different in its nature from any of its constituents: called distinctively chemical or elective affinity.
• noun In biology, morphological and implied genetic relationship, resulting in a resemblance in general plan or structure, or in the essential structural parts, existing between two organisms or groups of organisms; true and near structural relationship, predicable of two or more organisms morphologically related, however diverse physiologically.
• noun In psychology, that in ideas which renders them capable of being associated in the mind, as their similarity or coadjacency.
• noun In geometry, the relationship between two figures in the same plane which correspond to each other, point to point and straight line to straight line, any point of the one lying in a fixed direction from the corresponding point of the other, and at a distance from it proportional to its distance from a fixed line, called the axis of affinity, the direction of which is that of lines joining corresponding points.

from the GNU version of the Collaborative International Dictionary of English.

• noun Relationship by marriage (as between a husband and his wife's blood relations, or between a wife and her husband's blood relations); -- in contradistinction to consanguinity, or relationship by blood; -- followed by with, to, or between.
• noun Kinship generally; close agreement; relation; conformity; resemblance; connection.
• noun obsolete Companionship; acquaintance.
• noun (Chem.) That attraction which takes place, at an insensible distance, between the heterogeneous particles of bodies, and unites them to form chemical compounds; chemism; chemical or elective affinity or attraction.
• noun (Nat. Hist.) A relation between species or higher groups dependent on resemblance in the whole plan of structure, and indicating community of origin.
• noun (Spiritualism) A superior spiritual relationship or attraction held to exist sometimes between persons, esp. persons of the opposite sex; also, the man or woman who exerts such psychical or spiritual attraction.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A natural attraction or feeling of kinship to a person or thing.
• noun A family relationship through marriage of a relative (e.g. sister-in-law), as opposed to consanguinity. (e.g. sister).
• noun A kinsman or kinswoman of such relationship. Affinal kinsman or kinswoman.
• noun The fact of and manner in which something is related to another.
• noun Any romantic relationship.
• noun Any passionate love for something.
• noun taxonomy resemblances between biological populations; resemblances that suggest that they are of a common origin, type or stock.
• noun geology structural resemblances between minerals; resemblances that suggest that they are of a common origin or type.
• noun chemistry An attractive force between atoms, or groups of atoms, that contributes towards their forming bonds
• noun medicine The attraction between an antibody and an antigen
• noun computing tendency to keep a task running on the same processor in a symmetric multiprocessing operating system to reduce the frequency of cache misses

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a close connection marked by community of interests or similarity in nature or character
• noun (anthropology) kinship by marriage or adoption; not a blood relationship
• noun (biology) state of relationship between organisms or groups of organisms resulting in resemblance in structure or structural parts
• noun the force attracting atoms to each other and binding them together in a molecule
• noun inherent resemblance between persons or things
• noun (immunology) the attraction between an antigen and an antibody
• noun a natural attraction or feeling of kinship