e·pen·the·sis 
(ĭ-pĕnthĭ-sĭs)
(ĭ-pĕnthĭ-sĭs)n. pl. e·pen·the·ses (-sēz′)
The insertion of a sound in the middle of a word, as in Middle English thunder from Old English thunor.
[Late Latin, from Greek, from epentithenai, to insert : ep-, epi-, epi- + en-, in; see EN-2 + tithenai, to place; see dhē- in the Appendix of Indo-European roots.]
ep′en·thetic (ĕp′ĭn-thĕtĭk) adj.