Extemporaneous means spoken without preparation. The orator's performance was impressive, but only after we learn that his speech was extemporaneous did we realize the true depth of his talent.

Some politicians are so skilled that even their prepared remarks have an extemporaneous quality. As opposed to policy debates where teams know the resolution in advance, extemporaneous debate is judged on style and nimble argumentation, as the participants receive the resolution minutes before debating begins.

Definitions of extemporaneous

adj with little or no preparation or forethought

“an extemporaneous piano recital”
ad-lib, extemporary, extempore, impromptu, off-the-cuff, offhand, offhanded, unrehearsed
without preparation; not prepared for

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