-th

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Variant of -eth¹.
2. n. Act; process: spilth.
3. n. State; quality: dearth.
4. n. Used to form ordinal numbers: millionth.

Wiktionary

1. n. rare Forming nouns from verbs of action.
2. n. rare Forming nouns from adjectives.
3. n. Used to form the ordinal numeral when the final term of the spelled number is not "first", "second", or "third".
4. n. archaic a variant of -eth, used to form the archaic third-person singular of verbs