1: relationship by marriage2 a: sympathy marked by community of interest : kinship b (1): an attraction to or liking for something <people with an affinity to darkness — Mark Twain> <pork and fennel have a natural affinity for each other — Abby Mandel> (2): an attractive force between substances or particles that causes them to enter into and remain in chemical combination c: a person especially of the opposite sex having a particular attraction for one3 a: likeness based on relationship or causal connection <found an affinity between the teller of a tale and the craftsman — Mary McCarthy> <this investigation, with affinities to a case history, a psychoanalysis, a detective story — Oliver Sacks> b: a relation between biological groups involving resemblance in structural plan and indicating a common origin