epenthesis (uh-PEN-thu-sis) noun

Insertion of an extra sound into a word, e.g. fillum for film.

[From Late Latin, from Greek epentithenai, to insert : ep-, epi-, (in addition) + en- (in) thesis (to place), stem of tithenia (to put).]

Exhibit 1: the regrettably ubiquitous ‘ath-a-lete.’ Exhibit 2: ‘height-th.’ Another instance of it is el-lum for elm.