A signal consisting of a pivoted arm that can be in either of two positions, depending on the message to be conveyed. Now chiefly historical, having been largely replaced by electric colour light signals.
In railway signalling, a horizontal arm generally indicates danger, stop, caution, or the like; an arm angled upwards or downwards indicates clear, proceed.
3A signal conveyed by or as by semaphore; specifically each of a set of signals that can be represented by the position of a person's two outstretched arms, typically holding a flag in each.
Sometimes used to refer to other forms of signalling, such as flashing lights or the use of flags.
Early 19th century. From semaphore + signal.