An adonis is a very handsome man, especially a young one. You might secretly think of your good-looking neighbor as "a young adonis."
You'll most commonly see this word capitalized: Adonis, though if you're using it to mean "handsome guy," it's okay to use a lower-case a. Its origin is the god Adonis, in Greek mythology the eternally youthful god of beauty and desire. When you call someone an adonis, there's an implication that he's a bit vain about his good looks: "He knows he's the adonis of the family."