In corpus linguistics, a hapax legomenon (/ˈhæpəks lɪˈɡɒmɪnɒn/ also /ˈhæpæks/ or /ˈheɪpæks/; pl. hapax legomena; sometimes abbreviated to hapax, plural hapaxes) is a word or an expression 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 but more than once in that particular work. Hapax legomenon is a transliteration of Greek ἅπαξ λεγόμενον, meaning "said once".
The related terms dis legomenon, tris legomenon, and tetrakis legomenon respectively (/ˈdɪs/, /ˈtrɪs/, /ˈtɛtrəkɪs/) refer to double, triple, or quadruple occurrences, but are far less commonly used.