n (nautical) A flag flown by a ship to show the presence on board of the admiral; the admiral himself, or his flagship.
n (nautical|often used attributively) A signal flag.
n The use of a flag, especially to indicate the start of a race or other event.
n (context|computer science) A variable or memory location that stores a true-or-false, yes-or-no value, typically either recording the fact that a certain event has occurred or requesting that a certain optional action take place.
n (context|computer science) In a CLI | command line interface, a notation requesting optional behavior or otherwise modifying the action of the command being invoked.