giving or receiving of pleasure from acts involving the receipt or infliction of pain or humiliation
Picture from Aline et Valcour (1795) by De Sade. The caption reads Is heaven just when it abandons virtue to so big a torment..?

Sadomasochism is a word which is mostly used in a social (non-medical) way. It means that people get together to either give or receive pain and humiliation. Usually it's a form of sex play.

Sadists enjoy inflicting pain on others or humiliating them. Masochists get pleasure from being tortured, hurt, or humiliated.