In the Middle Ages, “quorum” referred specifically to commissions of justices of the peace. It was a part of a longer Latin phrase: “quorum vos unum [or ‘duos,’ etc., for the required number of members] esse voluminous,” meaning “of whom we wish that you be one [or two, etc].” The word “quorum” on its own has retained virtually the same usage. It refers to the number of members of a body required to be present in order to conduct business. Your book club may require a quorum of half the members to vote on the next month’s selection, for example. “Quorum” can also mean simply “a select group,” unrelated to voting.