adjective
spoken or done without preparation; impromptu
adverb
in an extemporaneous manner
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The speaker delivered an impressive extemporaneous speech on the importance of education.
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I was amazed by how well she could come up with extemporaneous jokes on the spot.
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I can't believe he just did that extemporaneous rap battle out of nowhere!
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Her extemporaneous decision to quit her job and travel the world changed her life forever.