Boos, known in Japan as Teresa (テレサ), are ghosts from the Mario and Yoshi series of video games. They first appeared as a common enemy in 1988's Super Mario Bros. 3, in which they were called Boo Diddleys (a reference to the American blues singer Bo Diddley). Since then, they have been a mainstay in the Mario series, usually appearing as enemies, but occasionally appearing as playable characters in the Mariospin-off games, or even as allies of Mario. They are named after the sound that one might make when trying to frighten someone ("boo!"), whereas their Japanese name comes from the verb 'tereru' (照れる), meaning 'to be shy'.
A Boo's most common pattern of attack is to sneak up on the player (usually Mario) from behind while they are facing away from them; however, they stop their attack if looked at. Being ghosts, they are usually invincible, and unable to be defeated without certain items or conditions.
It has been implied that Boos may have been other living species before they became boos, but this has not been made clear. There are also sub-species of Boo based on other species; for example undead Shy Guys become "Boo Guys" while undead Lakitus become "Fishin' Boo"s.
In an interview with Nintendo Power magazine, Mario franchise creator Shigeru Miyamoto stated that while working on Super Mario Bros. 3, co-designer Takashi Tezuka had the idea of putting his wife in the game. According to Miyamoto, "(Tezuka's) wife is very quiet normally, but one day she exploded, maddened by all the time he spent at work. In the game, there is now a character who shrinks when Mario looks at it, but when Mario turns away, it will grow large and menacing".
Boos appear as white, spherical, levitating ghosts, similar to the will-o'-the-wisp phenomenon or the Japanese Hitodama. They have faces featuring large fangs and long tongues, and three short extremities which resemble two arms and a tail. Being ghosts, Boos often appear in haunted house-themed levels in the games, in which they'll "sneak up" on the player, much in the fashion of ghosts of popular culture. Their presence can be recognized by their signature cackle. They are generally depicted as bashful and mischievous in nature.