What Does It Mean?

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From our dictionary:
sor•tie

(sôr'tE), n., v., -tied, -tie•ing.
—n.

1. a rapid movement of troops from a besieged place to attack the besiegers.
2. a body of troops involved in such a movement.
3. the flying of an airplane on a combat mission.
—v.i.
1. to go on a sortie; sally forth.