Improvisation

[Jason Yan]

The act or art of composing and rendering music,
poetry, and the like, extemporaneously; as, improvisation on the organ.

[Michiko]

To [make something up] on the spot.
[Improvisation] is common in jazz music, when a musician makes up a solo spontaneously, as opposed to playing a written solo.
Sometimes a group of musicians will completely [improvise] an entire song. This is done almost exclusively by jazz musicians.
All forms of art can be improvised. Art that is improvised may be called free or freestyle, like free poetry.
A popular form of [improvisation] is rapping, or flowing.

[Sommer]

The action of improvising.

[Agnes]

a performance that an actor, musician, etc. has not practised or planned: