A special-purpose data type introduced by E. W. Dijkstra that coordinates access to a particular resource or set of shared resources. A semaphore has an integer value (that cannot become negative) with two operations allowed on it. The signal (V or up) operation increases the value by one, and in general indicates that a resource has become free. The wait (P or down) 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. See also semaphore lock.