Salutation
A salutation is a greeting used in a letter or other written or non-written communication. Salutations can be formal or informal. The most common form of salutation in an English letter is Dear followed by the recipient's given name or title. For each style of salutation there is an accompanying style of complimentary close, known as valediction. Examples of non-written salutations are bowing (common in Japan) or even addressing somebody by their name. A salutation can be interpreted as a form of a signal in which the receiver of the salutation is being acknowledged, respected or thanked.
Another simple but very common example of a salutation is a military salute. By saluting another rank, that person is signalling or showing his or her acknowledgment of the importance or significance of that person and his or her rank. Some greetings are considered "rude" and others "polite".