Boo is the nickname of Arthur Radley, a character in the novel To Kill a Mockingbird.

Elementary school-aged child, a nickname perhaps derived from a character in the film Monsters, Inc.

Boo (Dragon Ball) (also known as "Buu"), a character in the manga Dragon Ball/Dragon Ball Z and the anime Dragon Ball Z.

Boötes, standard astronomical abbreviation

Kevin "Boo" McLee, an American football player

Boo! (TV series), a BBC children's television programme

A jeer, used to express one's distaste of a performance or action, especially in entertainment

An alternate spelling for Boô (a Saxon cattle shed).

A male rap artist, member of Young Money

Boo Weekley is an American golfer.

Boo! (1932 film), 1932 comedy short film

Boo! (Frasier episode), a 2004 episode of the television series Frasier

An exclamation in English used to evoke or suggest surprise, typically associated with ghosts.

A slang term for a significant other or boyfriend/girlfriend.

The pet miniature giant space hamster of Minsc in the Baldur's Gate role-playing computer game series.

Boo is a hamster and supporting character in Megatokyo, based on the pet of Minsc from Baldur's Gate.

Boo (programming language), an object-oriented, statically typed programming language for the Common Language Infrastructure.

boo.com, an infamous dot-com clothing company which went bust.

Better Off Out, a campaign for United Kingdom withdrawal from the European Union