In Judaism, the High Priest of Israel (; ) was the head of the Israelite priesthood. He played a unique role in the worship conducted in the Tabernacle and later in the Temple in Jerusalem, as well as in some non-ritual matters. Like all priests, he was required to be descended from Aaron (the first biblical priest). But unlike other priests, the high priest followed more restrictive laws, wore unique priestly garments, and was the only priest allowed to perform certain ceremonies.
Titles
The high priest is referred to by a number of titles in the Hebrew Bible; the title kohen gadol did not become dominant until well into the Second Temple period.
In addition to the title of "great priest" (kohen gadol) which later became the standard Hebrew title, the term "head priest" (kohen harosh) was used, as was "anointed priest" (kohen mashiach).
The Torah sometimes uses longer descriptions: "the great priest, who was anointed with the holy oil", "the priest who was anointed and invested to serve in place of his father" "the priest who is greater than his brothers, whom oil was poured on his head and who was invested to wear the garments", and "the priest in place of him [=Aaron] from his sons, who will come to the Tent of Meeting for the holy service".
Some verses refer by name to a specific individual who is understood to be "the" priest, i.e. the high priest at that moment.
Hebrew Bible
Individuals
The first high priest was Aaron himself, the ancestor of all priests; he was appointed to this role by God.
Other notable high priests in the Bible include Aaron's son Eleazar, Eleazar's son Phinehas, Eli, and Zadok.
After the Babylonian Exile, the first high priest was Joshua the High Priest, who is mentioned as a prominent leader alongside the political ruler Zerubavel. Joshua was a son of Zadok, in accordance with Ezekiel's prophecy.
It was presumed that the high priest's son would succeed him, but this did not always happen. The role eventually passed from Phinehas (or his descendants) to Eli, who was descended from Ithamar (brother of Eleazar). However, Eli's family eventually lost rights to the high priesthood due to their sins, a prophecy fulfilled when King Solomon appointed Zadok (a descendant of Eleazar) in place of Abiathar (a descendant of Eli).
Roles
The distinguished rank of the high priest is apparent from the fact that his sins are regarded as belonging also to the people. He was entrusted with the Urim and Thummim, which were worn in his garments.
On Yom Kippur he alone performed the Yom Kippur Temple service, entering the Holy of Holies to atone for himself and for the people. Only he could offer the sacrifices for the sins of the entire people. He also offered a meal-offering twice daily, on behalf of himself and the whole priesthood.
An unintentional killer was required to remain in the city of refuge until the death of the high priest, at which point the killer could return home.
Qualifications
The high priest alone was anointed with holy anointing oil upon his consecration, hence the title of the "anointed priest" (though in the initial consecration of Aaron and his sons, all of them were anointed). The consecration also required wearing the special high priestly vestments for seven days.
He was permitted to marry only an Israelite virgin.
He was required to strictly avoid ritual defilement. He was forbidden to come in contact with the bodies of the dead, not even those of his parents (unlike regular priests, who may become ritually impure for the death of an immediate relative). He was also forbidden to leave his hair dishevelled or exposed, or to rend his garments, as signs of mourning.
Vestments
The Torah provides for specific vestments to be worn by the priests when they are ministering in the Tabernacle. The high priest wore eight holy garments (bigdei kodesh). Of these, four were of the same type worn by all priests and four were unique to the high priest.
Those vestments which were common to all priests were the tunic (Hebrew ketonet), sash (avnet), turban (Hebrew mitznefet), and undergarments (michnasayim).
The vestments that were unique to the high priest were the robe (me'il), ephod (vest or apron), breastplate (hoshen), and headplate (Hebrew tzitz).
In addition to the above "golden garments", he also had a set of white "linen garments" (bigdei ha-bad) which he wore only on Yom Kippur for the Yom Kippur Temple service.