Source : Webster's Revised Unabridged Dictionary (1913)

Semaphore \Sem"a*phore\, n. [Gr. sh^ma a sign + fe`rein to bear:
   cf. F. s['e]maphore.]
   A signal telegraph; an apparatus for giving signals by the
   disposition of lanterns, flags, oscillating arms, etc.

Source : WordNet®

semaphore
     n : an apparatus for visual signaling with lights or
         mechanically moving arms

semaphore
     v 1: send signals by or as if by semaphore
     2: convey by semaphore, of information

Source : Free On-Line Dictionary of Computing

semaphore
     
         The classic method for
        restricting access to shared resources (e.g. storage) in a
        {multi-processing} environment.  They were invented by
        {Dijkstra} and first used in {T.H.E} {operating system}.
     
        A semaphore is a {protected variable} (or {abstract data
        type}) which can only be accessed using the following
        operations:
     
        	P(s)
        	Semaphore s;
        	{
        	  while (s == 0) ;	/* wait until s>0 */
        	  s = s-1;
        	}
     
        	V(s)
        	Semaphore s;
        	{
        	  s = s+1;
        	}
     
        	Init(s, v)
        	Semaphore s;
        	Int v;
        	{
        	  s = v;
        	}
     
        P and V stand for Dutch "Proberen", to test, and "Verhogen",
        to increment.  The value of a semaphore is the number of units
        of the resource which are free (if there is only one resource
        a "binary semaphore" with values 0 or 1 is used).  The P
        operation {busy-wait}s (or maybe {sleep}s) until a resource is
        available whereupon it immediately claims one.  V is the
        inverse, it simply makes a resource available again after the
        process has finished using it.  Init is only used to
        initialise the semaphore before any requests are made.  The P
        and V operations must be {indivisible}, i.e. no other process
        can access the semaphore during the their execution.
     
        To avoid {busy-wait}ing, a semaphore may have an associated
        {queue} of processes (usually a {FIFO}).  If a process does a
        P on a semaphore which is zero the process is added to the
        semaphore's queue.  When another process increments the
        semaphore by doing a V and there are tasks on the queue, one
        is taken off and resumed.
     
        (1995-02-01)