good afternoon

Also, good afternoon or evening or morning. Formal ways of saying "Hello" or "Goodbye." For example, He began rather oddly by addressing the audience with "Good day," or "Good afternoon, ladies," said the sales clerk as we walked out. All these greetings represent an abbreviation of the now obsolete God give you a good day (afternoon, etc.), which dates from about 1200. Also see good night.

The noun has one meaning:

Meaning #1: a conventional expression of greeting or farewell
  Synonym: afternoon