The Klingon High Council is led by an individual called the Chancellor. What title is given to the other members of the Council? "Councillor"? "Lord"? "Senator"??? I can only recall them being referred to collectively ("the council") or via a verb ("your brother sits on the council") but never addressed directly.
Based on a reading of the original screenplays available, the correct terms appears to be 'Councilor' (with a single 'l').
From TNG: Redemption.
K'TAL: (booming voice) Gowron, son of M'Rel, hakt'em.
Gowron steps forward... gives all the Councilors a hard look. Most of them glare back at him, unhappy to be admitting Gowron to their ranks.
One by one, they step down from the dais and stand behind Toral until only K'tal and the other Councilor (who voted against him) are left.
and from TNG: Reunion
COMPUTER VOICE: Council member Duras.
She reacts. A long beat.
K'EHLEYR: Access biographical files. Subject name: Councilor Duras.
It seems that "Councillor" is the most likely title. In the TNG episode "Reunion," after the computer calls Duras "Council member Duras," Ambassador K'ehleyr refers to him as "Councillor Duras."
K'EHLEYR: By whose order?
COMPUTER: Council member Duras.
K'EHLEYR: Access biographical files. Subject name, Councillor Duras.
The computer is obviously referring to Duras by position, while K'ehleyr is using his title.
This is backed up by the script for the Discovery episode "The Expanse." The introductory scene is set in the Klingon Council chamber and includes a brief conversation between the Chancellor, Duras and an unnamed Councillor:
CHANCELLOR: Twice, twice he's been captured, and twice he's escaped. Our Magistrate should never have shown him mercy. He should have been executed for his crimes.
COUNCILLOR: You had a simple mission, Duras. Locate the rebels that Archer was harbouring and return them to the Empire. But you failed. Archer made a fool of you.
CHANCELLOR: We are offering you a chance to regain your command, and your honour.
DURAS: I will not fail.