tooth fairy

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A fairy imagined to leave money under a young child's pillow in place of a tooth that has just fallen out.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A figure of modern myth said to give children a small amount of money (or sometimes a present) in exchange for a tooth when it falls out of a child's mouth.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a fairy that is said to leave money at night under a child's pillow to compensate for a baby tooth falling out