Boundary condition for index terms?

Thanks to thatwhichmatter, I came across the term "hapax legomenon". (In a quick search, I found that this is also the name of a character in the MMORPG City of Heroes.)

From the Greek, it literally means a word that has only been said (or used) once. Not just in an author's works, or in a collection. Once. A small collection of examples is here. And it seems to go across languages -- examples from ancient Greek, Chaucer, Shakespeare. How do these differ from neologisms, for example? Are they just failed neologisms?