1. an expresion of gratitude used primarily as a formal, friendly, social obligation.
2.When used in it's true form (not merely out of obligation) it is an expresion of respect and appreciation to another soul (and simultaneously to all beings) for the unconditional kindness that has been bestowed upon you.
1. (delivery person hands man package) man: "Thank you!"
2. (parents tolerate spoiled bratty kid for 20 years and teach kid how to not be so spoiled and bratty) Kid: "Thank you."