A type of crownless "hat" (which is domeless), consisting of an eyeshade and something to fasten it to the head, such as a rim or an elastic strap.
Not to be confused with peaked cap, which is associated with a police or military uniform.
Note that a visor cap does not meet Wiktionary's definition of a hat, because it is not closed at the top end. For the purposes of tagging, Danbooru does not recognize it as a hat, hense the lack of an implication.