The Collaborative International Dictionary
On the alert
Alert \A*lert"\, n. (Mil.)
An alarm from a real or threatened attack; a sudden attack;
also, a bugle sound to give warning. ``We have had an
alert.''
--Farrow.
On the alert, on the lookout or watch against attack or danger; ready to act.
Wiktionary
on the alert
a. Very attentive, or vigilant
WordNet
on the alert
adj. watchful and prepared for action; "bird watchers on the alert for a rare species"; "cops were on the job and caught them red-handed" [syn: on the alert(p), on the job(p)]