The Collaborative International Dictionary of English v.0.48:

Extemporization \Ex*tem`po*ri*za"tion\, n.
   The act of extemporizing; the act of doing anything
   extempore.
   [1913 Webster]

WordNet (r) 3.0 (2006):

extemporization
    n 1: a performance given extempore without planning or
         preparation [syn: extemporization, extemporisation,
         improvisation]