1.Definition: engaged in or accustomed to close observation

"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"

Related Adjective(s):watchful

2.Definition: mentally perceptive and responsive

"An alert mind", "Alert to the problems"

Related Adjective(s):alive, awake

3.Definition: quick and energetic

Related Adjective(s):brisk, lively, merry, rattling, snappy, spanking, zippy