http://en.m.wikipedia.org/wiki/Hapax_legomeno

A hapax legomenon (/ˈhæpəks lɨˈɡɒmɨnɒn/ also /ˈhæpæks/ or /ˈheɪpæks/;[1][2] pl. hapax legomena; sometimes abbreviated to hapax, pl. hapaxes) is a word that occurs only once within a context, either in the written record of an entire language, in the works of an author, or in a single text. The term is sometimes incorrectly used 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 ἅπαξ λεγόμενον, meaning “(something) said (only) once”.