adj. 1 awake, wide awake, watchful, vigilant, attentive, heedful, wary, cautious, on the qui vive, aware, on guard, on the lookout, observant, Colloq on the ball, on one's toes The sentinels must remain alert throughout the night Kenneth is alert to the perils of smoking cigarettes. 2 active, nimble, lively, agile, active, quick, spry, sprightly, vivacious He is an alert and joyous old soul
n. 3 lookout She is always on the alert for new ways of saving money 4 alarm, warning, signal, siren Sound the air-raid alert!
v. 5 warn, caution, advise, alarm, forewarn, signal, notify We must alert him to the fact that the man is a vicious killer