Definition from Wiktionary, the free dictionary
Alternative forms 
- Sometimes written as n'th or n-th
nth (not comparable)
- (mathematics) Occurring at position n in a sequence.
- (informal) Occurring at a relatively large but unspecified position in a series.
- 1922 — Edgar Rice Burroughs, The Chessmen of Mars, Prelude
- Shea had just beaten me at chess, as usual, and, also as usual, I had gleaned what questionable satisfaction I might by twitting him with this indication of failing mentality by calling his attention to the nth time to that theory, propounded by certain scientists, which is based upon the assertion that phenomenal chess players are always found to be from the ranks of children under twelve, adults over seventy-two or the mentally defective.
- (informal): umpteenth
occurring at position n
nth (plural nths)
- (mathematics) The item at position n in a sequence.
- (informal) The item in a relatively large but unspecified position in a series.
- 1909 — Bram Stoker, The Lady of the Shroud
- This is, after all, not strange, for none of them had either knowledge or experience in Occult matters, whereas for over thirty years I have made a special study of this subject, and have gone to and fro over the earth investigating to the nth all records of Spiritual Phenomena.