What is the difference between a prosthesis and an orthosis?

A prosthesis is an artificial replacement of a missing, amputated or incompletely formed body part.
An orthosis is an orthopaedic aid attached externally to the body that is intended to relieve, stabilise or correct joints, muscles or bones, complementing their form and function.
In short: prosthesis replaces missing body part, orthosis supports the form and function of a body part.