from Wiktionary, Creative Commons Attribution/Share-Alike License
- interj. An expression of gratitude or politeness, in response to something done or given.
- n. A specific expression of gratitude.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. a conversational expression of gratitude
Thank you is a shortened expression for I thank you; it is attested since c. 1400. (Wiktionary)