A minor second is an interval of one semitone.
The interval between C and Db, for example, is a minor second as it contains one semitone.
In most common scales, such as the major scale and the natural minor scale, the interval between the root on the scale and the second note on the scale is either one semitone or two semitones. The smaller interval is called a minor second (one semitone) and the larger interval is called a major second (two semitones). These intervals are called "second" if they occur between the first note and the second note on a scale (or more generally between two adjacent notes). A more rarely occurring interval between two adjacent notes is the augmented second (three semitones). (A "diminished" second is impractical, as it would be equal to zero semitones, or unison).