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 one

b :  a relation between biological groups involving resemblance in structural plan and indicating a common origin