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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.
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

At this point, Hernandez said, “The congresswoman was alert and conscious.”
Gabrielle Giffords’ Prognosis Hopeful After Shooting Casey Schwartz January 8, 2011

Hurst also spread the blame to police, who he said had been aware of the computer hacking but failed to alert him.
Murdoch’s Next Scandal Could Be Computer Hacking Mike Giglio February 20, 2012

That way maybe you can get some sleep, since if the law is closing in you can hope the dog will bark and alert you.
Advice to George Zimmerman From a Former Fugitive From Justice Mansfield Frazier April 10, 2012

The school guard, while unarmed, was still able to alert the authorities.
An American-Style School Shooting in Moscow Andrew S. Bowen February 3, 2014

Using a GPS system, the undergarments can also apparently send an alert to parents or police.
Shocking! Indian Engineers Introduce Electric ‘Anti-Rape’ Underwear Isabel Wilkinson April 2, 2013

Historical Examples

Whoever has done literary work is likely to have discovered how constantly the literary mind must be on the alert.
Talks on Writing English Arlo Bates

He sprang, now right, now left, on the alert for his opening.
The Rich Little Poor Boy Eleanor Gates

Mr. Glascock at once made a decided attempt to bolt; but the minister was on the alert, and was too quick for him.
He Knew He Was Right Anthony Trollope

He had been on the alert all the while, trying to catch what the governor and Sipiagin were saying.
Virgin Soil Ivan S. Turgenev

The attempt, however, was discovered before the fire gained any hold, and merely put the authorities on the alert.
German Society at the Close of the Middle Ages Ernest Belfort Bax

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.

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.
(1999-03-29)
Allergy to Latex Education and Resource Team
see: on the alert