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1550s, "addition of a letter or syllable to a word," from Late Latin, from Greek prosthesis "addition," from prostithenai "add to," from pros "to" + tithenai "to put, place" (see theme). Meaning "artificial body part" is first recorded c.1900, from earlier use to describe the medical art of making artificial limbs (1706), on notion of "that which is added to" the injured body.
prosthesis pros·the·sis (prŏs-thē'sĭs)
n. pl. pros·the·ses (-sēz)
An artificial device used to replace a missing body part, such as a limb or a heart valve.
Replacement of a missing body part with such a device.