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