Definitions:

1. remark made spontaneously without prior preparation noun
   • The candidates' adlibs got them in trouble with the politicians.
2. said or done without having been planned or written in advance adjective
   • The speaker opened with a few adlib remarks.
3. to perform without preparation verb
   • I adlibbed the performance.
4. with little or no preparation or forethought adjective
   • Their adlib comments showed poor judgment.

Synonyms:

• extemporise
• extemporize
• impromptu
• improvise
• improvize
• spontaneous
• unwritten