flag

A flag is a piece of cloth, usually marked with a colourful symbol or sign.

A flag flown by a ship is to show the presence on board of the admiral (the leader of a group of ships); the admiral himself; or his flagship.

The use of a flag, especially to show the start of a race or other event.

Example: Be ready to start running immediately at the flag.

On a computer, a flag is a variable or place in the computer's memory that keeps a true-or-false, yes-or-no value, usually either recording the fact that a certain event has happened or asking for a certain action to happen if it's an action that doesn't always happen.

In a command line interface (a place on a computer screen for people to type commands for a computer to do), a sign telling the computer to do something it does only sometimes, or a sign changing in some other way the action of the command being typed.