epenthesis

noun pl. -·ses′-sēz′

1. Phonet. a change which involves the insertion of an unhistoric sound or letter in a word, as the b in mumble or the extra syllable in the pronunciation (at̸hʹə·lēt) for athlete
2. the inserted sound or letter

Etymology: LL < Gr < epi-, upon + en-, in + thesis, a placing: see thesis