(1) A mechanism that is used to synchronise one or more jobs.
(2) An indicator used to control access to a file. For example, in a multiuser application, a semaphore is a flag that prevents simultaneous access to a file.
(3) In UNIX systems, a general method of communication between two processes that extends the features of signals.
(4) An object used by multi-threaded applications for signalling purposes and for controlling access to serially reusable resources. Processes can be locked to a resource with semaphores if the processes follow certain programming conventions.