A quorum is the minimum number of members of a deliberative assembly (a body that uses parliamentary procedure, such as a legislature) necessary to conduct the business of that group. According to Robert's Rules of Order Newly Revised, the "requirement for a quorum is protection against totally unrepresentative action in the name of the body by an unduly small number of persons." In contrast, a plenum is a meeting of the full (or rarely nearly full) body. A body, or a meeting or vote of it, is quorate if a quorum is present (or casts valid votes).

The term quorum is from a Middle English wording of the commission formerly issued to justices of the peace, derived from Latin quorum, "of whom", genitive plural of qui, "who". As a result, quora as plural of quorum is not a valid Latin formation. In modern times a quorum might be defined as the minimum number of voters needed for a valid election.