semaphore

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A visual signaling apparatus with flags, lights, or mechanically moving arms, as one used on a railroad.
2. n. A visual system for sending information by means of two flags that are held one in each hand, using an alphabetic code based on the position of the signaler's arms.
3. v. To send (a message) or to signal by semaphore.

Century Dictionary and Cyclopedia

1. n. A mechanical device for displaying signals by means of which information is conveyed to a distant point. The word is now confined almost entirely to apparatus used on a railways employing the block system. The blade is a day signal, the lantern is used at night. A vertical position of the blade or a white light exhibited by a lantern indicates safety; a horizontal position of the blade or a red light indicates danger; an intermediate position of the blade or a green light demands a cautious approach with lessened speed.
2. To signal with a semaphore; humorously, wave one's arms and signal like a semaphore.

Wiktionary

1. n. Any visual signaling system with flags, lights, or mechanically moving arms.
2. n. A visual system for transmitting information by means of two flags that are held one in each hand, using an alphabetic and numeric code based on the position of the signaler’s arms.
3. n. computing A bit, token, fragment of code, or some other mechanism which is used to restrict access to a shared function or device to a single process at a time, or to synchronize and coordinate events in different processes.
4. v. transitive, intransitive To signal using (or as if using) a semaphore.

GNU Webster's 1913

1. n. A signal telegraph; an apparatus for giving signals by the disposition of lanterns, flags, oscillating arms, etc.

WordNet 3.0

1. v. send signals by or as if by semaphore
2. v. convey by semaphore, of information
3. n. an apparatus for visual signaling with lights or mechanically moving arms