Baa is the bleating sound that sheep and lambs make. If you've ever visited a farm or petting zoo, you've probably heard sheep calling to each other with their distinctive “baa, baa” sound. The word imitates the actual noise, which is why it sounds similar in many languages.

Sheep baa for different reasons: a ewe (mother sheep) calls to find her lamb, lambs baa when they're hungry or want their mother, and adult sheep baa to communicate with their flock. Each sheep's baa sounds slightly different, and mother sheep can recognize their own lamb's voice among dozens of others.

The word appears in the nursery rhyme “Baa, Baa, Black Sheep” and often shows up in children's books about farms. When you baa at a sheep, you're trying to speak its language, though real sheep probably think you sound a bit silly.

As an interjection, baa! can also be used to imitate a sheep's sound.