In harmony, the tertium major is the ratio 5:4 between a pair of frequencies or, equivalently, the ratio 4:5 between a pair of wavelengths. It is the arithmetic mean of unison and diapente.
It is 1.01 in binary, and it is the inversion of the minor sixth (8:5), which means that it is equal to diapason divided by the minor sixth:
Three tertium majors are slightly less (-41.05 cents) than a diapason. A tertium major is also called a major third, but notice that here it has been defined as a simple ratio, such as is used in just intonation.
See also: diatessaron, tertium minor.