Used in technical and general contexts for the degree to which things combine, mix, or get along well. Common in chemistry, biology, and interpersonal descriptions.
This drug has a high affinity for a specific receptor.
I've never really been compatible with him.
affinity maturation
Compound of 親和 (shinwa, 'friendship, harmony') + 性 (sei, 'nature, property'). The exact historical derivation is uncertain, but the word is a modern technical coinage.