The musical interval of a Major third is the relationship between the first note (the root or tonic) and the third note in a major scale. It is the inversion of the minor sixth.
It can be produced by starting on a high note and playing the third below or by starting on a low note and playing the third above.
A Major third in just intonation corresponds to a pitch ratio of 5:4 or 1:1.25 while in an equal tempered tuning, a Major third is equal to four semitones, a ratio of 1:24/12 (approximately 1.259), or 400 centss, 13.686 cents larger.
In the common practice period thirds are considered the most interesting and dynamic consonance along with its inverse the sixthes, but in previous times it was considered an unusable dissonance. The Major third of a Major chord is what gives it its description as "happy", as opposed to the "sad" minor third of a minor chord. The Major third is considered the most consonant after the octave, perfect fifth, and the perfect fourth.