A nit is the tiny egg of a louse, a parasitic insect that sometimes lives in human hair. Nits are about the size of a pinhead and stick firmly to individual strands of hair near the scalp. They look like small white or tan dots and are notoriously difficult to remove. School nurses check for nits because head lice spread easily among children who share hats or sit close together.

The word appears in the common expression nitpicking, which means focusing on tiny, often unimportant details or flaws. When someone nitpicks your essay, they're pointing out every minor mistake instead of looking at the bigger picture. A nitpicker might complain about a crooked picture frame while ignoring that the whole room looks great. Just as finding actual nits requires examining hair strand by strand, nitpicking means examining something with excessive attention to trivial details.

In British English, people sometimes call someone a nit as a mild insult, similar to calling them silly or foolish.