body double

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A movie actor who substitutes for a leading performer, especially in distance shots or scenes not involving the face, such as close-ups of a portion of the body.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A film stand-in for an actor, in scenes not involving the face, especially during scenes of nudity, or of dangerous activity.
• noun A political decoy; a person employed to impersonate a politician, in order to draw attention away from the real person or to take risks on their behalf.