Other forms: kippahs
A kippah or yarmulke, is a kind of skullcap worn by Jewish men.
It's a part of Jewish custom to wear a kippah, but it's not always considered strictly necessary. It's worn primarily during prayer, although some Jewish men wear one at all times. Kippah is a Hebrew word and yarmulke is a Yiddish word, but they both refer to the same thing.