A melody or sequence of notes is diminished if the lengths of the notes are shortened (this is opposed to augmentation, where the notes are lengthened). A melody originally consisting of four crotchets (quarter-notes) for example, is augmented if it later appears with four quavers (eighth-notes) instead. This technique is often used in contrapuntal music. It gives rise to the "canon in diminution", in which the notes in the following voice are shorter than those in the leading.
An interval is diminished if it is narrowed; a diminished chord is one which contains a diminished interval. Thus a diminished fifth, for example, is a half step narrower than the perfect fifth, and a diminished chord is a minorr chord whose fifth note has been lowered a half step. The opposite is augmented.