Definition from
http://en.wiktionary.org

Improvisation (Noun)
The act or art of composing and rendering music, poetry, and the like,
[without prior preparation or planning].

Improvisation is a performance that is done on the spur of the moment.

No rules truly govern musical improvisation, but it must have some sort of structure based on music theory, else it will be unintelligible...a mess or blur of sound.

Musical improvisation is a skill that must be practiced.

A good place to start with practicing musical improvisation is to study theory.

Improvisation practicing is the same as you would practice anything else.

Practicing in front of an audience is definitely challenging, but essential.

Musical improvisation is a skill. Some musicians have devoted their entire life to this skill.