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Improvisation

Music created as it is played, usually without notation and unprepared. Often employed in genre such as Indian Music, Jazz, free jazz and sometimes in classical music in cadenza to concertos (the part where the soloist is completely on their own and the orchestra or accompanying group stops).

Interlude

A piece of music played between other pieces, e.g. between acts of a play; scenes of an opera; or between verses of a hymn on the organ. Some pieces of music are called interludes and have such qualities without the above connotations.

Introduction

Any musical material which precedes the main substance of a piece.

Inversion

In melody, literally turning the time 'upside down'.