A pixie is a mythical creature that resembles a mischievous fairy. If you spy some adorable little sprites in green pointed hats sneaking around the woods, you can bet that they're pixies.
Pixies appear in folk and fairy tales from the southwest part of England, making minor mischief and dancing in groups. Today a pixie is more or less the same as a fairy or sprite, but older folktales describe conflicts (and even wars) between these groups. The root of the word pixie is a mystery — it may be related to the Swedish dialect pyske, "small fairy," but it's probably rooted in an unknown Celtic word.