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alert: able to think clearly and to notice things

1
a : watchful and prompt to meet danger or emergency
b : quick to perceive and act
2
: active, brisk
— alert·ly adverb
— alert·ness noun

Examples of ALERT:
An alert guard stopped the robbers.
He was tired and had trouble staying alert while he was driving.
She wasn't mentally alert enough to answer the questions.
An alert watchdog guarded the door.
Origin of ALERT:
Italian all'erta, on the watch, literally, on the height
First Known Use: 1618

Synonyms
Argus-eyed, attentive, awake, observant, open-eyed, tenty (also tentie) [Scottish], vigilant, watchful, wide-awake, on guard, on one's toes, on the alert, on the ball, on the qui vive, on tiptoe
Antonyms
asleep
Related Words
alive, aware, conscious, sensitive; cognizant, heedful, keen, mindful, observing, regardful, sharp, sharp-eyed; hyperalert, hypervigilant, sleepless, wakeful; careful, cautious, chary, wary; prepared, ready
Near Antonyms
absent, absentminded, absorbed, abstracted, daydreaming, dazed, distracted, dreaming, dreamy, engrossed, faraway, insensible, oblivious, preoccupied; sleeping, unaware, unconscious, unknowing, unwitting; careless, heedless, inattentive, unheeding, unmindful, unthinking, unwary; unprepared, unready

2alert noun

: something (such as a message or loud sound) that tells people there is some danger or problem : an alarm or signal of danger

: the state of being ready for something you have been warned about (such as an attack)
Full Definition of ALERT
1
: a state of careful watching and readiness especially for danger or opportunity
2
a : an alarm or other signal of danger
b : an urgent notice
3
: the period during which an alert is in effect


Examples of ALERT
They sounded an alert when enemy planes were approaching the city.
Medical officials have put out an alert to hospitals to look out for the virus.
The government has issued a terrorism alert.
First Known Use of ALERT
1803
Related to ALERT
Synonyms
admonishment, admonition, alarm (also alarum), warning, caution, forewarning, heads-up, notice
Related Words
auguring, augury, forecasting, foretelling, predicting, prediction, premonition, presaging, prognosticating, prophecy (also prophesy), prophesying; apprising, informing, notification, notifying, tip-off; advice, counsel, guidance, recommendation, suggestion, tip; announcement, declaration
Near Antonyms
absentmindedness, abstraction, daydreaming, daze, distraction; absorption, engrossment, obliviousness, preoccupation; unawareness, unconsciousness; carelessness, heedlessness, inattention, inattentiveness, inobservance; unwariness

alert transitive verb

: to give (someone) important information about a possible problem, danger, etc. : to warn (someone)

: to make (someone) aware of something

Full Definition of ALERT
1
: to call to a state of readiness : warn
2
: to make aware of
Examples of ALERT
Several neighbors alerted the authorities when they noticed strangers acting suspiciously.
The governor alerted island residents that a hurricane was coming.
The teacher alerted the students that tests would be given the next day.
First Known Use of ALERT
circa 1868
Related to ALERT
Synonyms
advise, warn, caution, forewarn, wake
Related Words
augur, forecast, foretell, harbinger, predict, presage, prognosticate, prophesy; apprise, inform, notify, tip (off); admonish; bode, forebode (also forbode), foreshadow, foretoken, portend
Near Antonyms
imperil, risk

http://www.merriam-webster.com/dictionary/alert