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

 

vigilant

 adjective
adjective
BrE BrE//ˈvɪdʒɪlənt//
 
; NAmE NAmE//ˈvɪdʒɪlənt//
 
(formal)
 
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very careful to notice any signs of danger or trouble synonym alert, watchful A pilot must remain vigilant at all times. The thief was spotted by vigilant neighbours. The organization is ever vigilant for threats to the habitats of birds. You need to be vigilant against garden pests. Word Origin late 15th cent.: from Latin vigilant- ‘keeping awake’, from the verb vigilare, from vigil ‘awake’.
See the Oxford Advanced American Dictionary entry: vigilant