Sign, signal can both mean a motion, an action, a gesture, or a word by which a command or wish is expressed or a thought is made known.
Sign (see also SIGN 1 ) ( CHARACTER 1 ) is the general term that in itself carries no explicit connotations; it is used in reference to a bodily motion (as a shrug) or a gesture (as a beckoning) or an action (as a pantomime) by which one conveys a thought, a command, a direction, or a need to another with whom one either cannot communicate orally (as by reason of deaf-mutism, or lack of a common language, or distance) or does not wish to communicate orally (as from consideration of others or desire for secrecy).
Signal usually applies to a conventional and recognizable sign that typically conveys a command, a direction, or a warning.
Signal is also applied to mechanical devices which by operating lights, moving barriers, or sounding an alarm, take the place of a guard, a watchman, or a policeman.