Improvisation is the activity of making or doing something not planned beforehand, using whatever can be found. Improvisation, in the performing arts is a very spontaneous performance without specific or scripted preparation.

Musical improvisation (also known as musical extemporization) is the creative activity of immediate ("in the moment") musical composition, which combines performance with communication of emotions and instrumental technique as well as spontaneous response to other musicians. Sometimes musical ideas in improvisation are spontaneous, but may be based on chord changes in classical music and many other kinds of music. One definition is a "performance given extempore without planning or preparation." Another definition is to "play or sing (music) extemporaneously, by inventing variations on a melody or creating new melodies, rhythms and harmonies." Encyclopædia Britannica defines it as "the extemporaneous composition or free performance of a musical passage, usually in a manner conforming to certain stylistic norms but unfettered by the prescriptive features of a specific musical text. Improvisation is often done within (or based on) a pre-existing harmonic framework or chord progression. Improvisation is a major part of some types of 20th-century music, such as blues, jazz, and jazz fusion, in which instrumental performers improvise solos, melody lines and accompaniment parts.

from Wikipedia