chamber music

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

• noun Compositions traditionally intended for performance in a private room or small concert hall and written for an instrumental ensemble, such as a trio or quartet, with one player for each part.

from The Century Dictionary.

• noun Music, either instrumental or vocal, which is specially suited for performance in a small room: opposed to concert-music, and also to church music and operatic music.

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

• noun music A genre of classical music written for a small group of instruments

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

• noun serious music performed by a small group of musicians