epenthesis

Concise Oxford English Dictionary © 2008 Oxford University Press:
epenthesis /ɛˈpɛnθɪsɪs/

▶noun (pl. epentheses /-siːz/) the insertion of a sound or letter within a word, e.g. the b in thimble.

– derivatives
epenthetic adjective.

– origin C16: via late L. from Gk, from epentithenai ‘insert’.