What does alert mean?
The definition of alert
A*lert”, a.
Etym: [F. alerte, earlier à l’erte on the watch, fr. It. all’ erta on the watch, prop. (standing) on a height, where one can look around; erta a declivity, steep, erto steep, p. p. of ergere, erigere, to erect, raise, L. erigere. See Erect.]

1 Watchful; vigilant; active in vigilance.
2 Brisk; nimble; moving with celerity. An alert young fellow. Addison.

Synonyms
— Active; agile; lively; quick; prompt.

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.