Next after the twelfth: an ordinal numeral.Constituting any one of thirteen equal parts into which anything is divided.n. One of thirteen equal parts into which anything is divided.n. In early English law, a thirteenth part of the rents of the year, or of movables, or both, granted or levied by way of tax.n. In music, the interval, whether melodic or harmonic, between any tone and a tone one octave and six degrees distant from it; also, a tone distant by such an interval from a given tone; a compound sixth.