Acting President
From dKosopedia
According to the 25th Amendment to the Constitution, the Vice President becomes the Acting President when the President is "unable to discharge the powers and duties of his office". This determination may be made either by the President himself or by Congress. In the former case, the transfer is usually only temporary, until such time as the President resumes power; this is typically done, for instance, when the President will be undergoing surgery.
The concept of Acting President in the case of Presidential incapacitation is distinct from Presidential succession, in which case the Vice President actually becomes President following the President's death, resignation, or removal from office.