Alert, a.
Etymology: [F. alerte, earlier à l'erte on the watch, fr. It. all' erta on the watch, prop. (standing) on a height, where one can look around; erta a declivity, steep, erto steep, p. p. of ergere, erigere, to erect, raise, L. erigere. See Erect.]
1. Watchful; vigilant; active in vigilance.
2. Brisk; nimble; moving with celerity. An alert young fellow. Addison.
Syn. -- Active; agile; lively; quick; prompt.
1 other Webster entry under this spelling
Alert, n. (Mil.)
An alarm from a real or threatened attack; a sudden attack; also, a bugle sound to give warning. "We have had an alert." Farrow. On the alert, on the lookout or watch against attack or danger; ready to act.
Shades of meaning
Alert, ready, and wide-awake refer to a watchful promptness for action. Ready suggests thoughtful preparation; the wandering Indian is alert, the trained soldier is ready. Ready expresses more life and vigor than prepared. The gun is prepared; the man is ready. Prompt expresses readiness for appointment or demand at the required moment. The good general is ready for emergencies, alert to perceive opportunity or peril, prompt to seize occasion. The sense of brisk, nimble is the secondary and now less common signification of alert. Compare active; alive; nimble; vigilant.