The Collaborative International Dictionary of English v.0.48:

Valediction \Val`e*dic"tion\, n. [L., valedicere, valedictum, to
   say farewell; vale farewell (imperative of valere to be
   strong or well) + dicere to say. See Valiant, Diction.]
   A farewell; a bidding farewell. --Donne.
   [1913 Webster]
   [1913 Webster]

WordNet (r) 3.0 (2006):

valediction
    n 1: a farewell oration (especially one delivered during
         graduation exercises by an outstanding member of a
         graduating class) [syn: valediction, valedictory
         address, valedictory oration, valedictory]
    2: the act of saying farewell