Traditionally, a semaphore was an apparatus featuring colored lights and mechanical arms, which allowed for simple visual signals to be conveyed from a distance. Such a device was used, for example, to signal the driver of a train that the track ahead was clear for his train to proceed (or that it wasn't). Thus, a semaphore was a physical device that was used to effect mutual exclusion in certain real situations, most importantly involving railroads. The word is commonly used in such a sense even now outside the computing community.

In the 1960s, the problem of mutual exclusion in computing environments, where processors had to be restricted in their access to shared resources, arose. Edsger Dijkstra invented the now-classic method of using semaphores as a new approach to solving the problem. Dijkstra's semaphore is not a physical device with arms and lights; it rather is a protected variable...