A major sixth is an interval of nine semitones.
For example, the interval between C and A is a major sixth as it contains nine 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 sixth note on the scale is either eight semitones or nine semitones. The smaller interval is called a minor sixth (eight semitones) and the larger interval is called a major sixth (nine semitones). These intervals are called "sixth" if they occur between the first note and the sixth note on a scale (or more generally between two notes with four notes in between). More rarely occurring "sixth" intervals are the augmented sixth (ten semitones) and the diminished sixth (seven semitones).