The word Semaphore is used in a number of contexts. It is usually used for a visual signaling apparatus that is available with flags, lights or even arms. It is also used to refer to a visual system of sending information by using two flags in each hand.
It has a very old history. Semaphore tricks have been in use since ancient times. These include, signaling to incoming or outgoing ships through lights in the light tower. Signaling to airplanes through flags etc.