The smaller of the two diatonic intervals which encompass 2 degrees of the of the diatonic scale. The minor-2nd is synonymous with the diatonic semitone, and is one chromatic semitone smaller than the major-2nd. Here are some examples:
minor-2nd minor-2nd / \ / \ C D E F G A B C s s minor-2nd = s
In 12-edo, the minor-2nd encompasses 1 of the 12 equal semitones, and in this tuning there is no difference between it and the chromatic semitone. The minor-2nd is thus an interval in the range of 100 cents.
The minor-2nd always involves a change of letter-name and may or may not involve a change of accidental (as opposed to the chromatic semitone, which keeps the same letter-name but always involves a change of accidental). Here are some examples of minor-2nds:
Ab : Bbb A : Bb A# : B Ax : B# Bb : Cb B : C B# : C# Cb : Dbb C : Db C# : D Cx : D# Db : Ebb etc.
Narrowing the minor-2nd by one more chromatic semitone results in the diminished-2nd.
See the diatonic-semitone entry for more information.