Wordnet 3.0
NOUN (1)
1. 
 the insertion of a vowel or consonant into a word to make its pronunciation easier; 
- Example: "the insertion of a vowel in the plural of the word `bush' is epenthesis"
The Collaborative International Dictionary of English v.0.48:
Epenthesis \E*pen"the*sis\, n.; pl. Epentheses. [L., fr. Gr.
   ?; 'epi` + ? to put or set in.] (Gram.)
   The insertion of a letter or a sound in the body of a word;
   as, the b in "nimble" from AS. n[=e]mol.
   [1913 Webster]
WordNet (r) 3.0 (2006):
epenthesis
    n 1: the insertion of a vowel or consonant into a word to make
         its pronunciation easier; "the insertion of a vowel in the
         plural of the word `bush' is epenthesis"