The Collaborative International Dictionary of English v.0.48:

Prosthesis \Pros"the*sis\, n. [L., fr. Gr. ? an addition, fr. ?
   to put to, to add; ? to + ? to put, place.]
   1. (Surg.) The addition to the human body of some artificial
      part, to replace one that is wanting, as a log or an eye;
      -- called also prothesis.
      [1913 Webster]

   2. (Gram.) The prefixing of one or more letters to the
      beginning of a word, as in beloved.
      [1913 Webster]

WordNet (r) 3.0 (2006):

prosthesis
    n 1: corrective consisting of a replacement for a part of the
         body [syn: prosthesis, prosthetic device]