a·lert 
(ə-lûrt)
(ə-lûrt)adj.
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.
n.
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·lerts
Idiom: 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).]
a·lertly adv.
a·lertness n.