The definition of extemporaneous is something done or spoken with little or no preparation.(adjective)
An example of extemporaneous is "extemporaneous acting," when an actor practices their lines only once before a performance.
See extemporaneous in Webster's New World College Dictionary
Origin: LL extemporaneus: see extempore
See extemporaneous in American Heritage Dictionary 4
Origin: From Late Latin extemporāneus
Origin: , from Latin ex tempore; see extempore.
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