Any principal priest in the Jewish Council.
Baker Encyclopedia of the Bible
Tyndale Bible Dictionary
Eerdmans Dictionary of the Bible
A Dictionary of Christ and the Gospels: Aaron–Zion
CHIEF PRIESTS (ἀρχιερεῖς).—In the Gospels ἀρχιερεύς properly denotes the individual who for the time being held the office of Jewish high priest; and when the word occurs in its singular form, ‘high priest’ is the almost invariable rendering it receives throughout the NT, both in AV and RV (in Lk