1. adjective Of or characterized by an impromptu fashion, with little or no preparation beforehand. Jazz improv is always a bit ad-lib by design; you can't plan what you might play!

2. adverb Done in an impromptu or free-hand fashion, with little or no preparation beforehand. His comedy is always so unique; I hear he does the whole thing ad-lib.

3. noun Something, usually performative in nature, that is done with little or no preparation beforehand. His speech this morning felt like it was a bit of an ad-lib. I don't think he prepared any notes ahead of time.

4. verb To say, do, or perform something in an impromptu fashion, with little or no preparation beforehand. I had forgotten to rehearse my lines for the audition, so I just decided to ad-lib the whole way through.