\Thir"teenth`\, a. [From {Thirteen}: cf. AS.
[thorn]re['o]te['o][eth]a.]
1. Next in order after the twelfth; the third after the
   tenth; -- the ordinal of thirteen; as, the thirteenth day
   of the month.
2. Constituting or being one of thirteen equal parts into
   which anything is divided.

\Thir"teenth`\, n.
1. The quotient of a unit divided by thirteen; one of
   thirteen equal parts into which anything is divided.
2. The next in order after the twelfth.
3. (Mus.) The interval comprising an octave and a sixth.