Ad-lib and Improvise

I'm sure you've heard the expression "ad-lib." To ad-lib (verb) means "to improvise" or "to deliver spontaneously and the noun form is what is produced in that way. 

It is most commonly used with acting. That makes sense because ad-lib is a shortening of the Latin ad libitum, which means "in accordance with one's wishes." In its earliest usage, ad libitum referred to acting and any activity where the performer was free to do whatever they liked for as long as desired. 

The term was extended beyond performance to drawing, public speaking, singing or playing an instrument, playing a sport, doing an experiment, making a repair and many other situations. 

In performance, "improvisation" is a similar term and used more often than ad-lib. To improvise is to create and perform (particularly music or drama) spontaneously, without preparation or script or sheet music.

Improv or ad-lib?

The two terms are not exactly the same. I think of a performer such as Robin Williams. He was known for his improvisation. Was he sometimes ad-libbing? I learned that he often used bits that he had done before but it seemed like an ad-lib to audiences because it might be in a new context. Improvisation suggests something new, but you could ad-lib lyrics to a song if you forgot the actual lyrics or wanted to extend it.