A word or expression attested only once within a given corpus, author, or language record. Singular in the surviving evidence, and therefore fragile in interpretation. Where a rare word has company, a hapax legomenon is lonely by definition: its meaning must be inferred without siblings.

From Greek hapax "once" + legomenon "(something) said," from legein "to say." The phrase is scholarship's way of naming a particular kind of silence: the archive speaking only once, then refusing repetition.