The vigilant security guard caught the intruder immediately.
Synonyms for vigilant
watchful
alert
careful
Antonyms for vigilant
careless
heedless
inattentive
Practice vigilant on real SAT questions
Vocabulary like vigilant appears on the Digital SAT in Words-in-Context questions, where you choose the word that fits a passage's meaning and tone. You can also drill this word with vocabulary flashcards.