alert

UK: /əˈlɜː^rt/ US: /əˈlɝt/ ,(ə lûrt′)

WordReference Random House Learner's Dictionary of American English © 2025

a•lert /əˈlɜrt/   adj. 

1. paying complete attention;
   wide-awake:
   The children were alert, following the teacher's instructions.
2. watchful and ready to act;
   vigilant:
   The prison guard was alert at his post.
   [~ + to]
   remained alert to the danger.

n. [countable]

3. a warning or alarm of an impending military attack, a storm, etc.:
   a tornado alert.
4. the period in which such a warning or alarm is in effect.

v. [~ + object (+ to + object)]

5. to warn (troops, ships, etc.) to prepare for a coming attack:
   alerted the air squadron.
6. to make aware of;
   warn:
   alerting the townspeople to the danger.

Idioms

7. Idioms on the alert, prepared for danger or opportunity.

a•lert•ly, adv. 
a•lert•ness, n. [uncountable]

WordReference Random House Unabridged Dictionary of American English © 2025

a•lert  (ə lûrt′), adj. 

1. fully aware and attentive;
   wide-awake;
   keen:
   an alert mind.
2. swift;
   agile;
   nimble.

n. 

3. an attitude of vigilance, readiness, or caution, as before an expected attack.
4. a warning or alarm of an impending military attack, a storm, etc.:
   We'd just boarded the bus when the alert sounded.
5. the period during which such a warning or alarm is in effect.
6. 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.

v.t. 

7. to warn (troops, ships, etc.) to prepare for action.
8. to warn of an impending raid, attack, storm, etc.:
   The radio alerted coastal residents to prepare for the hurricane.
9. to advise or warn;
   cause to be on guard:
   to alert gardeners to the dangers of some pesticides.

• Latin ērigere to erect
• Italian all'erta, equivalent. to all(a) to, on the + erta lookout, watchtower, origin, originally feminine of erto, past participle of ergere
• 1590–1600; 1940–45 for def. 4;

a•lert′ly, adv. 
a•lert′ness, n. 
Synonyms

• 1. awake, wary, observant.
  Alert, vigilant, watchful imply a wide-awake attitude, as of someone keenly aware of his or her surroundings.
  Alert describes a ready and prompt attentiveness together with a quick intelligence:
  The tourist was alert and eager to see the sights.
  Vigilant suggests some immediate necessity for keen, active observation, and for continuing alertness:
  Knowing the danger, the scout was unceasingly vigilant.
  Watchful suggests carefulness and preparedness:
  watchful waiting.
• 2. brisk, lively, quick, active, sprightly, spirited.

Antonyms

• 1. unaware.

Collins Concise English Dictionary © HarperCollins Publishers::

alert / əˈlɜːt/ adjective (usually postpositive)

1. vigilantly attentive: alert to the problems
2. brisk, nimble, or lively

noun

1. an alarm or warning, esp a siren warning of an air raid
2. the period during which such a warning remains in effect
3. on the alert ⇒ on guard against danger, attack, etc
4. watchful; ready: on the alert for any errors

verb (transitive)

1. to warn or signal (troops, police, etc) to prepare for action
2. to warn of danger, an attack, etc

Etymology: 17th Century: from Italian all'erta on the watch, from erta lookout post, from ergere to build up, from Latin ērigere; see erect

aˈlertly adverb

aˈlertness noun