quartet

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

• noun A composition for four voices or four instruments.
• noun A group of four singers or four instrumentalists.
• noun A group of four.

from The Century Dictionary.

• noun In embryology, a group of four cleavage-cells considered as a unit, either because they arise at the same time, or because they have a similar origin or prospective function.
• noun In music: A composition or movement for four solo parts, either vocal or instrumental, usually without accompaniment.
• noun A company of four singers or players who perform quartets.
• noun In an orchestra the stringed instruments collectively, and in oratorio music the principal vocal soloists, are sometimes loosely called the quartet.
• noun A stanza of four lines.
• noun Same as quadruplet.
• noun The performers of such a composition, whether vocal or instrumental.

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

• noun A composition in four parts, each performed by a single voice or instrument.
• noun The set of four person who perform a piece of music in four parts.
• noun (Poet.) A stanza of four lines.

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

• noun music A music composition in four parts, each performed by a single voice or instrument.
• noun music The set of four musicians who perform a piece of music together in four parts.
• noun A group of four.

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

• noun four performers or singers who perform together
• noun four people considered as a unit
• noun the cardinal number that is the sum of three and one
• noun a set of four similar things considered as a unit
• noun a musical composition for four performers