The Collaborative International Dictionary of English v.0.48:

Sortie \Sor"tie\ (?; 277), n. [F., fr. sortir to go out, to
   issue, probably fr. L. sortus, for surrectus, p. p. of
   surgere to raise up, to rise up. See Source.] (Mil.)
   The sudden issuing of a body of troops, usually small, from a
   besieged place to attack or harass the besiegers; a sally.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sortie
    n 1: a military action in which besieged troops burst forth from
         their position [syn: sortie, sally]
    2: (military) an operational flight by a single aircraft (as in
       a military operation)