I. -n(t)th adjective
Etymology: Middle English threttenthe, alteration (influenced by threttene thirteen) of threttethe, from Old English thrēotēotha (akin to Old Norse threttāndi thirteenth), from thrēotīne, thrēotȳne, thrēotēne thirteen + -otha, -tha -th
1. : being number 13 in a countable series
the thirteenth day
— see number table
2. : being one of 13 equal parts into which anything is divisible
a thirteenth share of the money
II. noun
( plural thirteenths -n(t)s, -n(t)ths)
1. : number 13 in a countable series
the thirteenth of the month
2. : the quotient of a unit divided by 13 : one of 13 equal parts of anything
one thirteenth of the total
3. : a tax in medieval England comprising the 13th part of the value of movables or of an annual rent
4.
a. : the musical interval comprising an octave and a sixth
b. : a note or tone at this interval