Semaphore (lit. 'apparatus for signalling'; from Ancient Greek σῆμα (sêma) 'mark, sign, token' and Greek -φόρος (-phóros) 'bearer, carrier') is the use of an apparatus to create a visual signal transmitted over distance. A semaphore can be performed with devices including: fire, lights, flags, sunlight, and moving arms. Semaphores can be used for telegraphy when arranged in visually connected networks, or for traffic signalling such as in railway systems, or traffic lights in cities.

The Phryctoriae were a semaphore system used in Ancient Greece for the transmission of specific prearranged messages. Towers were built on selected mountaintops, so that one tower, the phryctoria, would be visible to the next tower, usually 30 km (20 mi) distant. Flames were lit on one tower, then the next tower would light a flame in succession.

The Byzantine beacon system was a semaphore developed in the 9th century during the Arab–Byzantine wars. The Byzantine Empire used a system of beacons to transmit messages from the border with the Abbasid Caliphate across Asia Minor to the Byzantine capital, Constantinople. The main line of beacons stretched over some 720 km (450 mi) with stations placed from 100 km (60 mi) to 60 km (35 mi). A message could be sent along the line in approximately one hour. A bonfire was set at the first beacon and transmitted down the line to Constantinople.

A smoke signal is one of the oldest forms of semaphore for long-distance communication. The smoke is used to transmit news, signal danger or gather people to a common area.

A signal lamp is a semaphore system using a visual signaling device, often utilizing Morse code. In the 19th century, the Royal Navy began using signal lamps. In 1867, then Captain, later Vice Admiral, Philip Howard Colomb for the first time began using dots and dashes from a signal lamp.

The modern lighthouse is a semaphore using a tower, building, or another type of structure designed to emit light from a system of lamps and lenses and to serve as a navigational aid for maritime pilots at sea or on inland waterways. Lighthouses mark dangerous coastlines, hazardous shoals, reefs, rocks, and safe entries to harbors; they also assist in aerial navigation. Originally lit by open fires and later candles, the Argand hollow wick lamp and parabolic reflector were introduced in the late 18th century. The source of light is called the "lamp" and the light is concentrated, by the "lens" or "optic". Whale oil was also used with wicks as the source of light. Kerosene became popular in the 1870s and electricity and carbide (acetylene gas) began replacing kerosene around the turn of the 20th century. Carbide was promoted by the Dalén light which automatically lit the lamp at nightfall and extinguished it at dawn. The advent of electrification and automatic lamp changers began to make lighthouse keepers obsolete. Improvements in maritime navigation and safety with satellite navigation systems like the Global Positioning System (GPS) have led to the phasing out of non-automated lighthouses across the world.

A flag semaphore is the telegraphy system conveying information at a distance by means of visual signals with hand-held flags, rods, disks, paddles, or occasionally bare or gloved hands. Information is encoded by the position of the flags. It is still used during underway replenishment at sea and is acceptable for emergency communication in daylight or using lighted wands instead of flags, at night.

International maritime signal flags are a system used by ships for communication at sea, each flag representing a specific letter, numeral, or message in the international maritime signal code. They are brightly coloured and easily distinguishable, serving purposes such as spelling out messages, identifying letters and numbers, signalling distress or requests for assistance and communicating nautical procedures. Standardized by the International Maritime Organization (IMO), these flags ensure global uniformity. They allow maritime professionals a tool for effective and clear communication at sea.