Part of speech: Adjective
engaged in or accustomed to close observation
Example: caught by a couple of alert cops; alert enough to spot the opportunity when it came; constantly alert and vigilant, like a sentinel on duty
Part of speech: Adjective
quick and energetic
Example: a brisk walk in the park; a lively gait; a merry chase; traveling at a rattling rate; a snappy pace; a spanking breeze
Part of speech: Adjective
mentally perceptive and responsive
Example: an alert mind; alert to the problems; alive to what is going on; awake to the dangers of her situation; was now awake to the reality of his predicament
Merry, Qui vive, Alarum, Snappy, Watchful
The word "alert" comes from Italian all'erta, which was borrowed from French alerte. Many other languages have similar words:
The Norman Conquest of 1066 brought thousands of French words into English overnight.