Thayer's Greek Lexicon

 1. chief priest, high priest

He above all others was honoured with the title of priest, the chief
of priests. It was lawful for him to perform the common duties of
the priesthood; but his chief duty was, once a year on the day of
atonement, to enter into the Holy of Holies (from which the other
priests were excluded) and offer sacrifices for his own sins and the
sins of the people, and to preside over the Sanhedrin, or Supreme
Council, when convened for judicial deliberations. According to
Mosaic law, no one could aspire to the high priesthood unless he
were of the tribe of Aaron and descended from a high priestly
family; and he on whom the office was conferred held it till death.
But from the time of Antiochus Epiphanes, when the kings of
Seleucideae and afterwards the Herodian princes and the Romans
arrogated to themselves the power of appointing the high priests,
the office neither remained vested in the pontifical family nor was
conferred on any for life; but it became venal, and could be
transferred from one to another according to the will of civic or
military rulers. Hence it came to pass, that during the one hundred
and seven years intervening between Herod the Great and the
destruction of the holy city, twenty eight persons held the
pontifical dignity. 2. the high priests, these comprise in addition
to one holding the high priestly office, both those who had
previously discharged it and although disposed, continued to have
great power in the State, as well as the members of the families
from which high priest were created, provided that they had much
influence in public affairs. 3. Used of Christ because by
undergoing a bloody death he offered himself as an expiatory
sacrifice to God, and has entered into the heavenly sanctuary where
he continually intercedes on our behalf.