a·lert (?-lûrt?)adj.1. Vigilantly attentive; watchful: alert to danger; an alert bank guard. See Synonyms at careful.2. Mentally responsive and perceptive; quick.3. Brisk or lively in action: the bird’s alert hopping from branch to branch.n.1. A signal that warns of attack or danger: Sirens sounded the alert for an air raid.2. A condition or period of heightened watchfulness or preparation for action: Nuclear-armed bombers were put on alert during the crisis.tr.v. a·lert·ed, a·lert·ing, a·lerts To notify of approaching danger or action; warn: a flashing red light that alerted motorists to trouble ahead.Idiom: on the alert Watchful and prepared for danger, emergency, or opportunity: bird watchers on the alert for a rare species.[French alerte, from Italian all’ erta, on the lookout : alla, to the, on the (from Latin ad illam, to that : ad, to; see ad- + illam, feminine accusative sing. of ille, that, the; see al- in Indo-European roots) + erta, lookout (from past participle of ergere, to raise, from Latin ?rigere; see erect).]a·lert?ly adv.a·lert?ness n.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.