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.alert (??l??t) adj (usually postpositive) 1. vigilantly attentive: alert to the problems. 2. brisk, nimble, or livelyn3. an alarm or warning, esp a siren warning of an air raid4. the period during which such a warning remains in effect5. on the alert a. on guard against danger, attack, etcb. watchful; ready: on the alert for any errors. vb (tr) 6. to warn or signal (troops, police, etc) to prepare for action7. to warn of danger, an attack, etc[C17: from Italian all’erta on the watch, from erta lookout post, from ergere to build up, from Latin ?rigere; see erect] a?lertly adv a?lertness nCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014a?lert (??l?rt) adj., n., v. a?lert?ed, a?lert?ing. adj. 1. fully aware and attentive; wide-awake. 2. quick to understand or respond; perceptive; acute. 3. watchful and ready to act; vigilant. n. 4. a warning of an impending military attack, a storm, etc. 5. the period such a warning or alarm is in effect. 6. a state of vigilance or readiness, as before an expected attack. v.t. 7. to warn to prepare for an attack. 8. to warn of an impending raid, storm, etc. 9. to cause to be on guard; warn: to alert gardeners to the dangers of some pesticides. Idioms: on the alert, ready, as for danger; vigilant. [1590?1600;