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fully aware and attentive; wide-awake; keen:
an alert mind.
swift; agile; nimble.
an att-tude of vigilance, readiness, or caution, as before an expected attack.
a warning or alarm of an impending military attack, a storm, etc.:
we’d just boarded the bus when the alert sounded.
the period during which such a warning or alarm is in effect.
to warn (troops, ships, etc.) to prepare for action.
to warn of an impending raid, attack, storm, etc.:
the radio alerted coastal residents to prepare for the hurricane.
to advise or warn; cause to be on guard:
to alert gardeners to the dangers of some pesticides.
on the alert, on guard against danger; in readiness; vigilant:
the state police are on the alert for an escaped convict believed to be in the area.
contemporary examples

he also alerted the wall street journal in an interview that was the first time i met sharon.
my moments with ariel sharon seth lipsky january 10, 2014

a few days later, a hunter spotted the burnt-out fiat and alerted authorities who discovered the macabre scene inside.
three-year-old coco is the italian mafia’s littlest victim barbie latza nadeau january 27, 2014

he has betrayed irina by saying too much in the message back to the circus; he alerted the russians to her.
‘tinker tailor soldier spy’: oscar nominees gary oldman and peter straughan lorenza muñoz february 14, 2012

after one user spotted footsteps in the snow near the edge of a cliff, tomnod alerted law enforcement.
now you can hunt for flight 370 abby haglage march 11, 2014

he alerted police and then raced across london in person to lend support, finding him at a police station.
william and harry hit rural pub for non-rager stag party tom sykes april 14, 2013

historical examples

did they scan sufficiently to identify anything, to be alerted by anything in any window, on the roof, or anyplace else?
warren commission (2 of 26): hearings vol. ii (of 15) the president’s commission on the -ss-ssination of president kennedy

he could feel the alerted tension of the four as the golden eyes glowed at him.
join our gang? sterling e. lanier

dr. charles gregory who had been previously alerted and then came in to take care of the wrist.
warren commission (4 of 26): hearings vol. iv (of 15) the president’s commission on the -ss-ssination of president kennedy

you see, mr. masterson’s alerted people at the commissioner’s office.
the best made plans everett b. cole

there were armed and alerted landing-parties around the grid itself, of course, but the capital city of dara lay open.
pariah planet murray leinster

adjective (usually postpositive)
vigilantly attentive: alert to the problems
brisk, nimble, or lively
noun
an alarm or warning, esp a siren warning of an air raid
the period during which such a warning remains in effect
on the alert

on guard against danger, attack, etc
watchful; ready: on the alert for any errors

verb (transitive)
to warn or signal (troops, police, etc) to prepare for action
to warn of danger, an attack, etc
adv.

“on the watch,” 1590s, from french alerte “vigilant” (17c.), from phrase à l’erte “on the watch,” from italian all’erta “to the height,” from erta “lookout, high tower,” noun use of fem. of erto, past participle of ergere “raise up,” from latin erigere “raise” (see erect). the adjective is attested from 1610s, the noun from 1803, and the verb from 1868. related: alerted; alerting.
allergy to latex education and resource team
see: on the alert