1
adj active, agile, attentive, awake, bright-eyed and bushy-tailed (informal) brisk, careful, circumspect, heedful, keeping a weather eye on, lively, nimble, observant, on guard, on one's toes, on the ball (informal) on the lookout, on the watch, perceptive, quick, ready, spirited, sprightly, vigilant, wary, watchful, wide-awake
2
n alarm, signal, siren, warning
3
vb alarm, forewarn, inform, notify, signal, warn
Antonyms
, adj careless, heedless, inactive, languid, lethargic, listless, oblivious, slow, unaware, unconcerned, unwary
n all clear
vb lull