A hapax legomenon (often abbreviated just to hapax) is a word which appears only once -- in a language, a single written work, or the entire body of work of a given author. According to Wikipedia: "Hapax legomenon is a transliteration of Greek ???? ?????????, meaning '(something) said (only) once.'"
Hapaxes are everywhere, particularly in historical texts -- indeed, many of the remaining untranslated hieroglyphs appear to be hapaxes, possibly because we lack enough text to see them more frequently -- or perhaps because they are unique words; it's hard to tell the difference. I briefly interned with a professor at Florida State who had a database of such terms of Mayan origin; sorting it by frequency revealed a depressingly large number of single-appearance terms.
Wikipedia gives us some good English examples, including one from Shakespeare: Honorificabilitudinitatibus, which can apparently be translated as "the state of being able to achieve honors"; obviously it's faster and easier just to say "honorificabilitudinitatibus," which, for the record, my spell-checker recognizes as a valid word. Honorificabilitudinitatibus appears only once in Shakespeare's work, in Love's Labour's Lost. W.P. Workman counted the number of hapaxes in various bodies of work (including Biblical works), showing significant variance in the number both of hapaxes per play, as well as hapaxes per page (Workman analyzed the Irving volumes of Shakespeare); here's a diagram showing some results:
Read more on hapaxes from Wikipedia, including this nugget of trivia from the Greek: "aphedron 'latrine' was a hapax legomenon thought to mean 'bowel' until an inscription was found in Pergamos." Ahem.