sig·nal 
(sĭgnəl)
(sĭgnəl)n.
1.
a. An indicator, such as a gesture or colored light, that serves as a means of communication. See Synonyms at gesture.
b. A message communicated by such means.
2. Something that incites action: The peace treaty was the signal for celebration.
3. Biology A physical entity, such as a chemical or an electromagnetic wave, that activates a cell receptor and elicits a specific response.
4.
a. Electronics An impulse or fluctuating quantity, as of electrical voltage or light intensity, whose variations represent coded information.
b. Computers A sequence of digital values whose variations represent coded information.
5. The sound, image, or message transmitted or received by means of telecommunications.
adj.
Notably out of the ordinary: a signal feat; a signal event.
v. sig·naled, sig·nal·ing, sig·nals or sig·nalled or sig·nal·ling
v.tr.
1. To make a signal to: I signaled the driver to proceed.
2. To relate or make known by signals: They have signaled their willingness to negotiate.
3. To cause an effect in (a cell) by the activation of a receptor, as by a neurotransmitter or hormone.
v.intr.
To make a signal or signals.
[Middle English, from Old French, from Medieval Latin signāle, from neuter of Late Latin signālis, of a sign, from Latin signum, sign; see SIGN.]
signal·er, signal·ler n.