noun pl. affinities -·ties

1. relationship by marriage
2. close relationship; connection
3. similarity of structure, as of species or languages, implying common origin; family resemblance
4. a natural liking or sympathy; esp., a mutual attraction between a man and a woman
5. a person of the opposite sex who especially attracts one
6. the attractive force, of varying strength for various elements, molecules, etc., that causes the atoms of certain elements to combine and stay combined

Etymology: ME affinite < OFr afinite < L affinitas < affinis, adjacent, related by marriage < ad-, to + finis, a border