From bowiki

Extracted from Musical Improvisation on Wikipedia

Musical improvisation (also known as Musical Extemporisation) 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. Thus, musical ideas in improvisation are spontaneous, but may be based on chord changes in Western music.

Because improvisation is a performative act and depends on instrumental technique, improvisation is a skill. There are musicians who have never improvised and other musicians who have devoted their entire lives to improvisation.

Solos are often regarded as being improvised whereas most that are successful are actually scripted