Definition of alert adjective from the Oxford Advanced Learner's Dictionary

alert

adjective
əˈlɜːt
 
; əˈlɜːrt
 
 
1 able to think quickly; quick to notice thingsSuddenly he found himself awake and fully alert.Two alert scientists spotted the mistake.Try to stay alert while driving at night.an alert mindmentally alert2 alert to something aware of something, especially a problem or dangerWe must be alert to the possibility of danger.She was alert to the slightest sound.alertly
əˈlɜːtli
 
; əˈlɜːrtli
 
adjective
alertness
əˈlɜːtnəs
 
; əˈlɜːrtnəs
 
noun [uncountable]