hapax legomenon (pl. legomena) is a Greek term for a word or form that occurs only once in the recorded corpus of a given language.
These are, perforce, in dead or, at best, poorly-documented languages. Old English has a fair number of these sole survivors, due among other reasons to the fact that although there was a lively generation of new coinages in the primarily oral language, the subsequent haphazard survival of written documents gives us only a random sample of the full language, hence, a number of hapax legomena.
Admittedly, I’m not using this term correctly, because the brief comment is not always about unique words. Rather, it’s about notable or interesting items.