A vigilant person is someone who is constantly alert and watchful. The term can be broken down into two crossword solutions: ARGUS (5 letters) and GUARD (5 letters). ARGUS is a reference to the Greek mythological figure of the same name, who was known for having 100 eyes and being able to see everything. GUARD is a reference to someone who is responsible for protecting or watching over something. Together, these two words describe a person who is always on the lookout and ready to protect.