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Synonyms for Ad-lib (same or very similar meaning)

WordNet sense 1 (said or done without having been planned or written in advance):
spontaneous, unwritten

WordNet sense 2 (not furnished with or using a script):

WordNet sense 3 (with little or no preparation or forethought):
extemporaneous, extempore, impromptu, off-the-cuff, offhand, offhanded, unrehearsed, extemporary, offhand

WordNet sense 4 (without preparation; not prepared for):

WordNet sense 5 (perform without preparation):
extemporize, improvise, extemporise, improvize

From the ODE community, based on WordNetadd/edit