A nonce word is one coined 'for the nonce'--made up for one occasion and not likely to be encountered again. When Lewis Carroll coined it, frabjous was a nonce word. Neologisms are much the same thing, brand-new words or brand-new meanings for existing words, coined for a specific purpose. Analogy, especially with familiar words or parts of speech, often guides the coiner, and occasionally these words will enter the standard vocabulary."
(Kenneth G. Wilson, The Columbia Guide to Standard American English. Columbia Univ. Press, 1993)