The Collaborative International Dictionary of English v.0.48:

Epithesis \E*pith"e*sis\, n. [NL., fr. Gr. ? a putting on; 'epi`
   upon + ? to place.]
   The addition of a letter at the end of a word, without
   changing its sense; as, numb for num, whilst for whiles.
   [1913 Webster]