General:
* Improvisation: This is the most common and direct noun form. It's often used to describe the act of improvising.
* Extemporization: This word is a bit more formal and suggests spontaneous speech or performance.
* Ad-libbing: This word is more informal and implies a spontaneous and often humorous change in a performance.
Specific to a particular context:
* Solution: If you're talking about improvising a solution to a problem.
* Performance: If you're talking about improvising a musical or theatrical performance.
* Design: If you're talking about improvising a design for something.
The best noun to use will depend on the specific context and the intended meaning.