A major seventh is an interval of eleven semitones.
For example, the interval between C and B is a major seventh as it contains eleven 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 seventh note on the scale is either ten semitones or eleven semitones. The smaller interval is called a minor seventh (ten semitones) and the larger interval is called a major seventh (eleven semitones). These intervals are called "seventh" if they occur between the first note and the seventh note on a scale (or more generally between two notes with five notes in between). A more rarely occurring "seventh" interval is the diminished seventh (nine semitones). (An "augmented seventh" is not practical as it would be equal to twelve semitones, which is just the same note an octave higher).