Post-coital means happening after sex. Doctors, researchers, and a certain kind of magazine writer use it; it is not a word you need at the dinner table. You will see it in post-coital contraception — the pill or device used after sex — and, more softly, in the post-coital quiet or glow people talk about. Though actually, the word is cooler than the moment it names. It describes the time after the act, not the feelings. If you want to speak plainly with friends, “after sex” is clearer and less stiff. This is the medical and formal cousin.