semaphore

1. Any equipment used for visual signalling by means of flags, lights, or mechanically moving arms, which are used to represent letters of the alphabet, or words.
2. A visual system for transmitting information using the above equipment; especially, by means of two flags held one in each hand, using an alphabetic and numeric code based on the position of the signaller's arms; flag semaphore.
3. 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.

   The thread increments the semaphore to prevent other threads from entering the critical section at the same time.

verb

To signal using, or as if using, a semaphore, with the implication that it is done nonverbally.