The terms perfect, major, minor, diminished, and augmented are used to describe the quality of an interval. Perfect is used with unisons, fourths, fifths, and octaves. Major and minor is used with seconds, thirds, sixths and sevenths. Diminished and augmented are used with all intervals.
Perfect intervals are labeled with an upper case "P."
Major intervals are labeled with an upper case "M."
Minor intervals are labeled with a lower case "m."
Diminished intervals are labeled "d", "dim." or "deg. or " o."
Augmented intervals are labeled "A", "Aug." or "+."
Examples: P1, P4, m2, m6, M3, M7, d3, deg.5, dim. 5, A6, Aug. 6, +6.