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early 15c., in reference to certain eminent justices of the peace, from Latin quorum "of whom," genitive plural (masc. and neuter; fem. quarum) of qui "who" (see who). The traditional wording of the commission appointing justices of the peace translates as, "We have also assigned you, and every two or more of you (of whom [quoram vos] any one of you the aforesaid A, B, C, D, etc. we will shall be one) our justices to inquire the truth more fully." The justices so-named usually were called the justices of the quorum. Meaning "fixed number of members whose presence is necessary to transact business" is first recorded 1610s.
The minimum number of members of a committee or legislative body who must be present before business can officially or legally be conducted. In the United States Congress, for example, either house must have a majority (218 in the House of Representatives, 51 in the Senate) to have a quorum.