b: skilled at or given to extemporaneous utterance
c : happening suddenly and often unexpectedly and usually without clearly known causes or relationships
"a great deal of criminal and delinquent behavior is…extemporaneous — W. C. Reckless"
2 : provided, made, or put to use as an expedient : makeshift
— ex·tem·po·ra·ne·ous·ly adverb
— ex·tem·po·ra·ne·ous·ness noun
One of my students was inclined to give extemporaneous piano performances, though she often said she played "at random." At the time, I suggested she use "I played an improvised piano piece" (suggesting no prior preparation) but she could say: "I played an extemporaneous piano piece" if there were some preparation ahead of time...