Definition of "ad lib"

ad lib

/æd ˈlɪb/

adverb
comparative more ad lib, superlative most ad lib
1. At pleasure.
2. At will.
3. To whatever extent.
  Quotations
  For one thing I had a splendid supper when I got on board—a whack of cold, lean beef and pighells, bread, butter ad lib., tea, and plenty of good bread.
  1951, Katherine Mansfield, Letters to John Middleton Murry, 1913-1922
4. Extemporaneously.
adjective
comparative more ad lib, superlative most ad lib
1. Extemporaneous, impromptu.
verb
third-person singular simple present ad libs, present participle ad libbing, simple past and past participle ad libbed