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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

And on the edges of evangelicalism, where alertness to “New Age” influence runs high, concern has bloomed into outrage.
The Strange Saga of ‘Jesus Calling,’ The Evangelical Bestseller You’ve Never Heard Of Ruth Graham February 22, 2014

Historical Examples

There was a fox-like cunning in the face and the sharp little eyes, and also alertness and malice.
The Facts Concerning The Recent Carnival Of Crime In Connecticut Mark Twain (Samuel Clemens)

They praised his eyes’ alertness, the smoothness of his muscles.
Buried Cities, Part 2 Jennie Hall

Janet was always galvanized into alertness here, Faber Street being no place to dream.
The Dwelling Place of Light, Complete Winston Churchill

The only change in it that they could notice was an accession of alertness.
White Fang Jack London

There was that about him which gave evidence of alertness and courage of the highest order.
Personal Recollections of a Cavalryman J. H. (James Harvey) Kidd

With the alertness of an Indian she heard the crackling of twigs in the underbrush.
Tess of the Storm Country Grace Miller White

They make a great display of activity and alertness, said Pennington, with disdain.
The Young Continentals at Bunker Hill John T. McIntyre

The effect of alertness which he always produced was more evident now than ever.
The Winning Clue James Hay, Jr.

There was swiftness of action, alertness of mind, and with these a complete absence of hurry or confusion.
The Sky Pilot in No Man’s Land Ralph Connor

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
n.

1714, from alert + -ness.
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