In music, improvisation is a process in which a major portion of the resultant performance is not prearranged or anticipated. It is generated by the creator(s) during a performance. The degree to which improvisation is important varies between different musical styles, but in jazz it is virtually always central. The nature and definition of improvisation are complex issues which have been discussed in detail elsewhere. For the particularly technical purposes of the present book, it is noteworthy that improvisation commonly involves a redrawing of the assumed ground of a musical form, through instrumental technique, just as much as by musical vocabulary or structure. As John Corbett entertainingly puts it:

Jazz is a music of the twentieth century which originated in the USA from Afro-American traditions. It achieves improvised intensification by rhythmic repetition and cyclic harmonic repetition. Much jazz involves 4/4 metre, with considerable accentuation of the constituent pulses, and a vital concern with syncopation, of a subtle and complex kind. The syncopation is not simply displacing strong beats to their half-point or by whole beats, but also making irregular displacements, often in complex subdivisions of triplets and quintuplets, and accompanied by large, abrupt and transient variations in dynamic and/or emphasis: these are components of the special rhythmic ‘swing’ inherent in most jazz.