Less, smaller. (1) intervals are said to be minor when they contain one semitone less than major. (2) A scale is said to be in the minor mode when its third and sixth are minor. Formerly minor music was described as "with the lesser third." (3) Flute minor, Klein flute, s small flute-stop on the organ, or 4 ft. or 2 ft pitch. (Stainer, John; Barrett, W.A.; A Dictionary of Musical Terms; Novello, Ewer and Co., London, pre-1900)
Just as we have the Major 2nd, 3rd, 6th, 7th and 9th so we also have the Minor 2nd, 3rd, 6th, 7th and 9th.
The Major interval above and right is six steps between the C and A. The Minor Interval to the lower right is marked slightly less or smaller than six steps with the flat sign indicating the A is flattened or slightly lowered in pitch from its natural pitch when found in the Major Scale of C.
The minor mode or scale form is actually the Aeolian mode of medieval times; the Aeolian mode and the Ionian mode (our present major scale) are the only medieval modes which have been in general use since the Renaissance. The minor mode creates a different impression from the major, because of the difference in the arrangement of the whole and half steps. There are three forms of the minor scale, and each has its own function. These are the natural or pure minor, the harmonic minor, and the melodic minor. (Brye, Joseph; Basic Principles of Music)
Natural minor has half steps between two and three and between five and six. Harmonic minor has half steps between two and three and between seven and eight, ascending; in descending it usually is natural.