A hapax legomenon is a word

Word

In language, a word is the smallest free form that may be uttered in isolation with semantic or pragmatic content . This contrasts with a morpheme, which is the smallest unit of meaning but will not necessarily stand on its own...

 which occurs only once within a context, either in the written record of an entire language, in the works of an author, or just in a single text. The term is sometimes used incorrectly to describe a word that occurs in just one of an author's works, even though it occurs more than once in that work. Hapax legomenon is a transliteration of Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 ἅπαξ λεγόμενον, meaning "(something) said (only) once".

The related terms dis legomenon, tris legomenon, and tetrakis legomenon refer respectively to double, triple, or quadruple occurrences, but are far less commonly used.

Hapax legomena are quite common, as predicted by Zipf's law, which states that the frequency of any word in a work or corpus is inversely proportional to its rank in the frequency table.