Definition: (adjective) Spoken, carried out, or composed with little or no preparation or forethought.
Synonyms: ad-lib, extemporaneous, impromptu, off-the-cuff, offhand, offhanded, unrehearsed.
Usage: The class performed an extempore skit that condensed the complex plot of Shakespeare's <i>Hamlet</i> into a single, 15-minute act.
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Source: The Free Dictionary