1. Improvisation: Ad-lib most commonly refers to spontaneous and unscripted speech or action during a performance, especially in theater, comedy, or music. It's the ability to think on your feet and create something new in the moment.
Example: "The actor forgot his lines, so he had to ad-lib for a few minutes until he remembered."
2. Ad-libbed: This is used as an adjective to describe something that was not planned or rehearsed but was added in during a performance.
Example: "The band's ad-libbed guitar solo was a highlight of the concert."
In both cases, the key element is spontaneity and improvisation.