1. able to think in a clear and intelligent way
2. paying attention to what is happening and ready to react quickly if necessary
Origin and usage
The adjective alert came into English via French from an Italian phrase ‘all’erta’ meaning ‘on the lookout’. It was first used in the late 16th century.
The adjective alert has two meanings, listed above. Alertness is a general quality and also a response to a specific situation: if you are alert to something you are aware of it and ready to react quickly to it. Alert is also a noun and a verb. To alert someone is to warn them about a possible danger or problem so they can take steps to deal with it or avoid it. An alert is a warning that something dangerous has happened or is going to happen. The noun is often preceded by another noun that tells you what the danger is; so a flood alert is a warning that a flood is coming and a weather alert is a warning about possible bad weather. A red alert is a warning that something very dangerous is likely to happen. Someone who is on the alert is paying attention to what is happening and ready to react quickly if necessary, while someone who is on full alert is ready to take action to deal with a dangerous situation. Less urgently, an alert is also a signal of some kind, for example on your phone, that reminds you to do something or attracts your attention. A spoiler alert is a warning that a spoiler is coming up.
“You little girls, when you grow up, must be on the alert to recognise your prime at whatever time of your life it may occur.”
(Muriel Spark, The Prime of Miss Jean Brodie)
focused, attentive, intent, vigilant, watchful