What does improvisation mean in the context of music?

Improvisation in the context of music refers to the act of creating music spontaneously, often in response to the moment or the surrounding musical context. This can involve musicians drawing on their knowledge, skills, and instincts to produce melodies, harmonies, or rhythms without any prior planning or written score. It's a key component in various music genres, including jazz, where musicians engage in spontaneous creation to express emotions or to interact dynamically with other performers.

The other concepts mentioned, such as composing with a plan, arranging pre-composed material, or performing set pieces, focus on structured approaches to music-making which do not embody the essence of improvisation. In improvisation, the emphasis is on the instantaneous nature of creation and the unique contributions each performance can yield.