hapax legomenon
Definition - A word or a phrase of which:
(1) There is only one recorded instance in a whole literature or
(2) There is only one recorded instance in an author's complete works.

1. If a word is used twice, it is called a dis legomenon; if three times, a tris legomenon; and if four times, a tetrakis legomenon.
2. Hapax legomena are common. Zipf's law predicts that about 40% to 60% of the words in a corpus are hapax legomena, and another 10% to 15% are dis legomenon. Thus, in the Brown Corpus of American English, about half of the 50,000 words are hapax legomena within that corpus.

Example -
(1) The word flother, a synonym for snowflake, is a hapax in sense (1) of the word: There is only one recorded instance in pre-1900 written English (from a circa 1275 manuscript).
(2) The word honorificabilitudinitatibus is a sense (2) hapax: It's only found once in Shakespeare's complete works (in Act 5, Scene 1 of Love's Labour's Lost).

Etymology -
In Greek the word literally means "(thing) said once."

Oxford English Dictionary -
Its first citation is from 1654:
"'Tis hapax legomenon read only here: and hence this variety of interpretations."
(John Trapp, Commentary on the 12 Minor Prophets , p. 605)

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