quorum

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The minimal number of officers and members of a committee or organization, usually a majority, who must be present for valid transaction of business.
• noun A select group.
• noun The minimal density of bacterial cells that results in altered gene expression in a population of bacteria.

from The Century Dictionary.

• noun In England, those justices of the peace whose presence is necessary to constitute a bench.
• noun 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.
• noun Requisite materials.

from the GNU version of the Collaborative International Dictionary of English.

• noun Such a number of the officers or members of any body as is competent by law or constitution to transact business

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.
• noun A selected body of persons.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a gathering of the minimal number of members of an organization to conduct business