What is improv?
Improvisation is the practice of acting, singing, talking and reacting, of making and creating, in the moment and in response to the stimulus of one's immediate environment. This can result in the invetion of new thought patterns, new practices, new structures or symbols, and/or new ways to act.
Philosophically, improvisation often focuses on bringing one's personal awareness "into the moment," and on developing a profound understanding for the action one is doing. This fusion of "awareness" and "understanding" brings the practitioner to the point where he or she can act with a range of options that best fit the situation, even if he or she has never experienced a similar situation.
To "extemporize" or "ad lib" is basically the same as improvising. Colloquial terms such as "let's play it by ear," "take it as it comes," and "make it up as we go along" are all used to describe "improvisation." (Wikipedia.com)