The definition of vigilant is watchful and alert.
An example of something vigilant is a guard dog.
staying watchful and alert to danger or trouble
Origin of vigilantFrench ; from Classical Latin vigilans, present participle of vigilare, to watch ; from vigil, awake: see vigil
On the alert, as for danger or error; watchful. See Synonyms at careful.
Origin of vigilantMiddle English, from Old French, from Latin vigilāns, vigilant-, present participle of vigilāre, to be watchful; see vigilante.
(comparative more vigilant, superlative most vigilant)