|Origin:||alerte, from Italian all' erta 'on the watch'|
giving all your attention to what is happening, being said etc:
The animal raised its head, suddenly alert.
Taking notes is one of the best ways to stay alert in lectures.
able to think quickly and clearly:
Jack was as mentally alert as a man half his age.
to know about or understand something, especially a possible danger or problem:
The authorities should have been alert to the possibility of invasion.
—alertness noun [uncountable]