In medicine, a prosthesis is an artificial body part meant to replace one that's missing. If you were born with only one arm, doctors might fit you with a prosthesis to fill in for your missing arm.
When someone loses a body part because of illness or an accident (or if they're born without a limb or another body part), they sometimes have the option to get a prosthesis. Some prostheses are very simple duplicates of the absent part, while others are extremely complex devices. Some limb prostheses are even "robotic," allowing their users to move them as if they were real. In Greek, prosthesis means "addition."