early 15c., "pertaining to the faculty of sight;" also "coming from the eye or sight" (as a beam of light was thought to do), from Late Latin visualis "of sight," from Latin visus "a sight, a looking; power of sight; things seen, appearance," from visus, past participle of videre "to see" (see vision). Meaning "perceptible by sight" is from late 15c; sense of "relating to vision" is first attested c. 1600. The noun meaning "photographic film or other visual display" is first recorded 1944.
late 14c., "visible sign, indication" (a sense now obsolete), also "a supernatural act of God; a device on a banner," from Old French signal, seignal "seal, imprint, sign, mark," from Medieval Latin signale "a signal," from Late Latin signalis (adj.) "used as a signal, pertaining to a sign," from Latin signum "identifying mark, sign" (see sign (n.)).
The restricted sense of "conventional or agreed-upon sign" (to commence or desist, etc.) is from 1590s. The meaning "modulation of an electric current" is from 1855, later applied to electromagnetic waves, hence the use of signal in radio (later television) broadcasting (1923). The railroad signalman is attested by 1840.
"remarkable, striking, notable," 1640s, an irregular adoption (by influence of the noun) from French signalé, past participle of signaler "to distinguish, signal" (see signal (n.)). The notion is "serving as a sign."