quorum

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The minimal number of officers and members of a committee or organization, usually a majority, who must be present for valid transaction of business.
2. n. A select group.

Century Dictionary and Cyclopedia

1. n. In England, those justices of the peace whose presence is necessary to constitute a bench. Among the justices of the peace it was formerly customary to name some eminent for knowledge and prudence to be of the quorum; but the distinction is now practically obsolete, and all justices are generally “of the quorum.”
2. n. The number of members of any constituted body of persons whose presence at or participation in a meeting is required to render its proceedings valid, or to enable it to transact business legally. If no special rule exists, a majority of the members is a quorum; but in a body of considerable size the quorum may by rule be much less than a majority, or in a smaller one much more. Forty members constitute a quorum or “house” in the British House of Commons.
3. n. Requisite materials.

Wiktionary

1. n. The minimum number of members required for a group to officially conduct business and to cast votes, often but not necessarily a majority or supermajority.
2. n. A selected body of persons.

GNU Webster's 1913

1. n. Such a number of the officers or members of any body as is competent by law or constitution to transact business

WordNet 3.0

1. n. a gathering of the minimal number of members of an organization to conduct business