Active in vigilance; watchful; vigilantly attentive.Moving with celerity; brisk; active; nimble: as, “an alert young fellow,” Addison, Spectator, No. 403.Lively, agile, quick, prompt, ready, spry.n. An attitude of vigilance; watch; guard: especially in the phrase on or upon the alert, upon the watch; on the lookout; guarding against surprise or danger: as, “the readiness of one on the alert,” Dickens.