valediction
Concise Oxford English Dictionary © 2008 Oxford University Press:
valediction /ˌvalɪˈdɪkʃ(ə)n/
▶noun the action of saying farewell.
■ a farewell.
– origin C17: based on L. vale ‘goodbye’ + dicere ‘to say’, on the pattern of benediction.
'valediction' also found in these Oxford entries:

