01 a. Watchful; vigilant; active in vigilance.
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1.
Watchful; vigilant; active in vigilance.
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2.
Brisk; nimble; moving with celerity.“An alert young fellow.” — Addison.
Entry
3 senses · 2 variants
Webster, 1913
Alert
02 n. An alarm from a real or threatened attack; a sudden attack; also, a bugle sound to give warning.
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1.
An alarm from a real or threatened attack; a sudden attack; also, a bugle sound to give warning.(Mil.)
Phrases & compounds
On the alert —
on the lookout or watch against attack or danger; ready to act.