Often abbreviated to "thanks". One of the most overused (trivially) and insufficient phrases in the entire english language. (I love you, said without *True* feeling/meaning is the first). I wish there was a better phrase for it. We say "thank you", the same words when someone passes the salt to us or holds a door open for us as we do when they've just saved your life or found your beloved dog who had been missing for a month. We say 'thank you' when someone tells us we got a phone call please return it and we say 'thank you' to a loved one who has done things so amazing for us we can barely comprehend, much less express them... One word, too many meanings...