jam session

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

• noun An informal gathering of musicians to play improvised or unrehearsed music.
• noun Informal An impromptu discussion.

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

• noun music An informal gathering of musicians to play music, especially improvised jazz or a similar genre.

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

• noun an impromptu jazz concert