Epenthesis is defined as the insertion of a sound––generally, a consonant between two other consonants in a cluster––that is the result of a historical change in language. English has two unusual cases of epenthesis in morphophonemic alternations, both cases involving the sound n intervocalically (between vowels), viz. (1) after the indefinite article a before words beginning with a vowel, e. g., an apple (cf. a napkin); (2) before the head word either and its constituent conjunction or of the construction neither . . . nor (cf. either . . . or).
In the last twenty or thirty years, even otherwise careful writers and speakers are to be observed making the mistake of dropping the epenthetic n of nor, witness the following sentence penned by a Canadian writer in a contemporary scholarly publication: “What should be clear, however, is that Peirce’s praise of Spinoza is neither careless nor inconsistent with his thought or [instead of correct nor], indeed, with the early twentieth-century development of pragmatism.” In this example, where or appears instead of nor, the mistake could be mitigated, of course, by the fact that neither has already been copied once to its complementary first [n]or, thereby freeing the second or from obligatory epenthesis. But it is a grammatical mistake nonetheless.
[TERMINOLOGICAL CLARIFICATION: Although the term epenthesis does service for insertion of a sound at any position of a word, it is generally reserved for medial position, whereas the term prothesis is more particularly used for insertion in initial position, as in neither and nor. The insertion of a sound at the end of a word is called paragoge, which means that the n of an is, strictly speaking, paragogic.]