The raising or lowering of a tone by a half-step, from its diatonic value in a chord. In Jazz usage, the fifth and ninth may be raised (augmented) or lowered (diminished); the fourth (or eleventh) may be augmented; the thirteenth may be diminished. The expression 'diminished seventh' is used solely as the name of a chord. Of course, in general music theory, any interval may be augmented or diminished.