An augmented interval is a semitone larger than a major or perfect interval.

A diminished interval is a semitone smaller than a minor or pefect interval.

Note that since the interval between two pitches cannot be smaller than zero, there can be no such thing as a diminished unison. The table below shows you whether an interval is diminished (d), minor (m), major (M), augmented (A), or perfect (P) based on the number of semitones (row) and generic interval (column.)