What is a Salutation?
An expression used to greet a person.
You might begin the body of a letter with the words "Dear Sir", "Dear Mr Smith" or "Dear John". All of these are salutations, of differing levels of formality.
When you greet a friend in the street with "Hello John!" or "Hi there!" that is a form of salutation.
The form of salutation varies enormously according to who is greeting whom, in what kind of circumstance.
When the telephone was first invented it created a great social quandary: How should you greet the caller when you pick up the phone if you did not know who it might be?
Thomas Edison is credited with suggesting the use of "Hello".