Other forms: overheard; overhearing; overhears
When you accidentally hear part of a private conversation, you overhear it. If you overhear your friends discussing the surprise party they're throwing for your birthday, you'll have to pretend to be surprised.
Some restaurants have tables so close together that it's hard not to overhear what your neighbors are talking about. And so many people walk around talking loudly on their mobile phones that it's common to overhear several conversations (or, at least one half of them) just walking down the sidewalk. This is a modern phenomenon, but overhear is an old word, from the sixteenth century idea of hearing "beyond the normal range of the voice."