improvisation

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

• noun The act or art of improvising.
• noun Something improvised, such as a musical passage or comedic skit.

from The Century Dictionary.

• noun The act of improvising; the act of composing poetry or music extemporaneously.
• noun A product of extemporaneous composition; an impromptu poem or musical performance.

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

• noun The act or art of composing and rendering music, poetry, and the like, extemporaneously.
• noun That which is improvised; an impromptu.

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

• noun The act or art of composing and rendering music, poetry, and the like, extemporaneously; as, improvisation on the organ.
• noun That which is improvised; an impromptu.
• noun Musical technique, characteristic of blues music.
• noun The act of improvising, acting or going about something without planning ahead

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

• noun a creation spoken or written or composed extemporaneously (without prior preparation)
• noun an unplanned expedient
• noun a performance given extempore without planning or preparation