noun (n)

• a creation spoken or written or composed extemporaneously (without prior preparation)(noun.artifact)

  source: wordnet30
• an unplanned expedient(noun.act)

  Synonym:
  temporary expedient
  

  source: wordnet30
• a performance given extempore without planning or preparation(noun.act)

  Synonym:
  extemporisation, extemporization
  

  source: wordnet30
• The act or art of composing and rendering music, poetry, and the like, extemporaneously; as, improvisation on the organ.(noun)

  source: webster1913