Hapax legomenon

Etymology

Greek, "(a thing) said once": "hapax" (once) plus "legomenon" (said, the passive participle of "legein," to say). Long used by classicists and biblical scholars; often clipped to just "hapax."

noun phrase · /HAP-aks lih-GOM-uh-non/
A word or form that appears only once in a given text, author, or body of writing.
What it names
The scholar’s lonely find — the single-occurrence word whose meaning must be guessed from one context alone, a recurring puzzle in ancient and sacred texts.
Example
The precise meaning of that Homeric adjective is disputed, since it is a hapax legomenon — it never appears anywhere else in Greek.
Related idea
Distinguish it from a nonce word, deliberately coined for a single occasion; a hapax legomenon merely happens, by accident of survival, to occur only once in the record.