I found myself still a bit confused on the difference between a hapax and a nonce-word, so I asked for an explication from Jesse Sheidlower, U.S. editor of the OED; his timely reply:

A hapax legomenon--the term is normally used only in reference
to dead languages--it a word or form that is found only once in
a given corpus (an entire language, the works of a particular
author, etc.). That's it--there's no other implication here about
its use. Often the assumption is that the word could have been
more common but the one example is all we have. Note that the
reoccurrence of the word in criticism doesn't change its status;
that is, if you say "so-and-so is a hapax in Shakespeare", your
use of "so-and-so" does not mean that you now have another
example in English and it's no longer a hapax.

A nonce-word is generally used to mean a word coined for a
specific occasion, with the implication that it's not likely to
be used again or outside a very limited range. While nonce-words
_can_ become widespread, the assumption is that they won't.