1. Vigilantly attentive; watchful: alert to danger; an alert bank guard. See Synonyms at careful.
2. Mentally responsive and perceptive; quick.
3. Brisk or lively in action: the bird's alert hopping from branch to branch.
1. A signal that warns of attack or danger: Sirens sounded the alert for an air raid.
2. A condition or period of heightened watchfulness or preparation for action: Nuclear-armed bombers were put on alert during the crisis.
tr.v. a·lert·ed, a·lert·ing, a·lertsIdiom:
To notify of approaching danger or action; warn: a flashing red light that alerted motorists to trouble ahead.
on the alert
Watchful and prepared for danger, emergency, or opportunity: bird watchers on the alert for a rare species.
[French alerte, from Italian all' erta, on the lookout : alla, to the, on the (from Latin ad illam, to that : ad, to; see AD- + illam, feminine accusative sing. of ille, that, the; see al-1 in the Appendix of Indo-European roots) + erta, lookout (from past participle of ergere, to raise, from Latin ērigere; see ERECT).]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
The American Heritage Dictionary Blog
Check out our blog, updated regularly, for new words and revised definitions, interesting images from the 5th edition, discussions of usage, and more.