A special-purpose data type introduced by E. W. Dijkstra (1965). Apart from creation, initialization, and annihilation, there are only two operations on a semaphore: wait (P operation or down operation) and signal (V operation or up operation). The letters P and V derive from the Dutch words used in the original description.

A semaphore has an integer value that cannot become negative. The signal operation increases the value by one, and in general indicates that a resource has become free. The wait operation decreases the value by one when that can be done without the value going negative, and in general indicates that a free resource is about to start being used. This therefore provides a means of controlling access to critical resources by cooperating sequential processes.