fully aware and attentive; wide-awake; keen: an alert mind.
swift; agile; nimble.
an attitude 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.
verb (used with object)
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.

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

alertly, adverb
alertness, noun
hyperalert, adjective
unalerted, adjective

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.

1. unaware. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
alert (əˈlɜːt)
1.  vigilantly attentive: alert to the problems
2.  brisk, nimble, or lively
3.  an alarm or warning, esp a siren warning of an air raid
4.  the period during which such a warning remains in effect
5.  on the alert
 a.  on guard against danger, attack, etc
 b.  watchful; ready: on the alert for any errors
6.  to warn or signal (troops, police, etc) to prepare for action
7.  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 ērigere; see erect]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

c.1600, from Fr. alerte "vigilant," from phrase à l'erte "on the watch," from It. all'erta "to the height," from erta "lookout, high tower," orig. fem. of erto, pp. of ergere "raise up," from L. erigere "raise." The adv. is the oldest form in Eng.; the adj. is from 1712, the noun from 1803, and
the verb from 1868.
Online Etymology Dictionary, © 2010 Douglas Harper
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Computing Dictionary

alert definition

operating system
/*'l*rt/ An audible and/or visual message intended to inform a system's users or administrators about a change in the operating conditions of that system or about some kind of error condition. In a graphical user interface, an alert would typically be displayed as a small window containing the message and a button to click to dismiss the window.

The Free On-line Dictionary of Computing, © Denis Howe 2010
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American Heritage
Abbreviations & Acronyms
Allergy to Latex Education and Resource Team
The American Heritage® Abbreviations Dictionary, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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American Heritage
Idioms & Phrases


see on the alert.

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
They can map out physical dangers and act as early-warning systems to alert
  humans to the presence of hazardous chemicals or fire.
So alert me about the models that are working on your campus or that you think
  should be tried.
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Bob held the stage with his actor's baritone and theatrical chuckle, his eyes
  crinkled and sharply alert.
Idioms & Phrases
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