A major second is an interval of two semitones.
For example, the interval between C and D is a major second as it contains two semitones.
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" interval is not practical, as it would simply be equal to zero semitones – the same note).