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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.
Historical Examples

Men who had clung through the hours to their saddles by sheer will power alone, tightened their lines and were alertly alive.
Ride Proud, Rebel! Andre Alice Norton

Braybrooke looked suddenly surprised and then alertly observant.
December Love Robert Hichens

“Probably he was the one who sent the letter,” said Penny alertly.
Clue of the Silken Ladder Mildred A. Wirt

After a pause, Sam alertly remarked, We have not kept a lookout.
An Oregon Girl Alfred Ernest Rice

Every slightest turn had to be alertly chosen, and the tangle of branches and vines made going by the stars nearly impossible.
The Cavalier George Washington Cable

He had the appearance of alertly listening, listening, for what?
The Apartment Next Door William Andrew Johnston

She was now staring with wide, eager eyes down the street, alertly watching the approach of anyone who came from that direction.
The Husbands of Edith George Barr McCutcheon

The lords of the Church nodded, looked sleepily or alertly benevolent.
Foes Mary Johnston

She paid no attention at first to the advancing hunters, save to alertly elude the many efforts to cast a noose over her head.
True Tales of Arctic Heroism in the New World Adolphus W. Greely

Possibly the uncle may have some suspicion of why she is moving about so alertly, and besides looking so pleased like.
Gwen Wynn Mayne Reid

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.

1787, from alert + -ly (2).
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