bodydouble

A body double is a general term for someone who substitutes for the credited actor of a character in any recorded visual medium, in shots where the character's body is shown but the face is either not visible or shown indistinctly, or in shots where the image of the credited actor's face is joined, usually by digital image processing, to the image of the body double's body. Body doubles are most often used for shots involving nudity. More specific terms are often used in special cases; a stunt double is used for dangerous or sophisticated sequences. This is in contrast to a stand-in who replaces an actor for non-filming purposes such as scene arrangement and lighting adjustments.
Stunt double or body double can both be used for cases where special skills are needed—anything from playing the piano, to competitive skiing. (text nicked from Wikipedia bodydouble photo by me)