Kippa
From Speakeasy
A kippa is a little beanie worn by Jewish guys. The smaller it is, the sillier it looks. It is often attached to the hair with a small hairclip to keep it from slipping off the head. The plural term for kippa is kippot. The kippa is a modern invention, and bears no resemblance to the headwear of the Israelites. The ancient Jews wore a headwear similar in style to keffiyeh and turbans now associated with Arab people.
Kippot on non-Jews
Gentiles, especially politicians, sometimes wear a kippa to Jewish events, synagogues, or visiting some special place in Israel as a sign of respect for the Jewish religion. Perhaps they are under the mistaken impression it is a requirement in Judaism. It isn't, as there is no scripture requiring it. It is basically a Jewish cultural tradition like matzo ball soup or usury.
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Catholic clergymen (bishops - including the Pope) wear a zucchetto (Italian for little pumpkin), which is a skull cap similar to a kippa. Unlike the Jewish kippa, a zucchetto has a small "stem" at the top for gripping, which explains the name.
Some Muslim men wear a skull cap called a kufi. These take different shapes depending on culture, but they are almost always larger than the Jewish kippa.