adjective

1. watchful and ready, as in facing danger
2. quick in thought or action; active; nimble

Etymology: Fr alerte, earlier à l'erte < It all' erta, on the watch < alla, at the + erta, a lookout, high (point) < VL *ergere, for L erigere, to erect

noun

1. a warning signal as of an expected air raid; alarm
2. the period during which such a warning is in effect

transitive verb

1. to warn to be ready or watchful the troops were alerted
2. to inform or make aware of we alerted them to their duties