Note: Do not rely on this information. It is very old.
Quorum originally denoted those justices of the peace in whose absence trials could not be held. The expression arose through the manner in which commissions were worded; it is stated (in Latin) that the body in question is to consist of certain members, quorum unum. A. B. esse volumus ("of whom we will that A. B. be one"), or some such phrase being added. The term now merely denotes the number of persons belonging to any body who must be present in order that business may be transacted.