adj., n., & v.
adj. 1 watchful or vigilant; ready to take action. 2 nimble (esp. of mental faculties); attentive.
n. 1 a warning call or alarm. 2 a warning of an air raid. b the duration of this.
v.tr. (often foll. by to) make alert; warn (were alerted to the danger). on the alert on the lookout against danger or attack. alertly adv. alertness n.
[ F alerte f. It. all' erta to the watch-tower ]