Hapax legomenon

A hapax legomenon (pl. hapax legomena, though sometimes called hapaxes for short) is a word that occurs only once in the corpus of a language. (Occurrence in a dictionary that cites the occurrence in the corpus does not count.) Some of these are misspellings; others are real words that are rare enough that they only got used once. If a word is used twice it is a dis legomenon, thrice tris legomenon. Beyond tetrakis legomenon, a word isn't rare enough to call it that.

Some examples: "Founday" is a metallurgy term for a six-day period. Why there is a word for this is not clear.