Source : Webster's Revised Unabridged Dictionary (1913)

Afternoon \Aft"er*noon"\, n.
   The part of the day which follows noon, between noon and
   evening.

Source : WordNet®

afternoon
     n 1: the part of the day between noon and evening; "he spent a
          quiet afternoon in the park"
     2: a conventional expression of greeting or farewell [syn: {good
        afternoon}]