A kippa, kippah or yamaka is a brimless cap or skullcap, made of cloth. Kippas (kippot in Hebrew) are worn by Jews to fulfill the customary requirement held by Orthodox authorities that the head be covered at all times.
The tradition to wear a kippa is not derived from the Bible itself. It is a custom which evolved as a sign of the Jewish recognition that there is someone “above” who watches every act.
Most synagogues and Jewish funeral services keep a supply of kippas ready for the temporary use of visitors who have not brought one themselves.
There is much debate among Jewish theological authorities as to whether or not wearing a kippa is required at all times. According to the Rambam, Jewish law says that a man is required to cover his head during prayer but there is no mention that a Jew is required to cover his head at all times.
However, according to some authorities it has since taken on the force of law because it is an act of 'Kiddush Hashem' (sanctification of the name of God).
The 17th-century authority Rabbi David HaLevi Segal (The Taz) suggested that the reason was to distinguish Jews from non-Jews. He wrote that nowadays wearing a yarmulke is required by law.
Other halachic authorities like Sephardi posek, the Chida (Rabbi David Yosef Azulai) say that wearing a head covering is an additional measure of piety. Former Sephardic Chief Rabbi of Israel Ovadia Yosef ruled that a kippa should be worn to show affiliation with the religiously observant community.
The Talmud states, 'Cover your head in order that the fear of heaven may be upon you.' Rabbi Hunah ben Joshua never walked 4 cubits (6.6 feet, or 2 meters) with his head uncovered. He explained: 'Because the Divine Presence is always over my head.'
The argument for the kippa has two sides. The Vilna Gaon says one can make a blessing without a kippah, since wearing a yarmulke is only a 'midos chassidus' ('exemplary attribute').
According to Rabbi Isaac Klein, a Conservative Jew ought to cover his head when in the synagogue, at prayer or sacred study, when engaging in a ritual act, and when eating. In the mid-19th century, Reformers led by Isaac Wise completely rejected the kippas after an altercation in which Rabbi Wise's yarmulke was knocked off his head. There is still great debate about whether or not wearing a kippa is Halachic law or simply a custom. Many Sephardic Jews only wear a kippah when praying and eating but otherwise go without one.