Imitation: A fugal technique of composition, in which one part introduces a theme, or subject which is then answered by the other parts by reitorating the same theme later in the work. This term is usually applied to fugal passages in choral music.
Imperfect Cadence: See half-cadence.
Impressionism: A stylistic period of composition that sought to put to music only the most immediate, direct impressions, upon the composer, of a given subject. Impressionism avoided traditional harmony of thirds, employing more often quartal or quintal tonality.
Impromptu: A short, improvisational-sounding piece.
Incidental Music: Short musical segments that accompany, or highlight dramatic moments in, a play, or other stage work.
Interlude: Short music used to bridge the acts of a play, or the verses of a hymn.
Introduction: The preparatory section, movement, or phrase of a musical work.
Interval: The distance between two notes, in terms of occilations per second. The difference in one half-step is about 35 beats per second.
Introit: "Entrance". A psalm sung at the start of the Roman Catholic Mass.
Invention: A short, contrapuntal piece.
Inversion: The different forms that a chord may take by changing the chord member that is the bass of the chord.
Invertible Counterpoint: counterpoint in which two or more voices can be interchanged for one another.
Ionian Mode: A medieval mode whose scale pattern is that of playing C to C on the white keys of a piano. This scale is identical to a major scale.