From WordNet (r) 3.0 (2006) [wn]:
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}, afternoon]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Afternoon \Aft"er*noon"\, n.
The part of the day which follows noon, between noon and
evening.
[1913 Webster]