—n. 1. an apparatus for conveying information by means of visual signals, as a light whose position may be changed.
2. any of various devices for signaling by changing the position of a light, flag, etc.
3. a system of signaling, esp. a system by which a special flag is held in each hand and various positions of the arms indicate specific letters, numbers, etc.
—v.t., v.i. to signal by semaphore or by some system of flags.