The United States Constitution grants certain powers only to the federal government. What are these powers called? 

The correct term for powers such as these is "enumerated
powers."  These are the powers that are given specifically to the Congress in Article I
of the Constitution.  These are powers that may only be used by the federal government
and are forbidden to the states.  An example of this would be the power to make treaties
with foreign countries.  The opposite of this is "reserved powers."  Reserved powers are
those powers that are specifically kept for the states in the Constitution.  These are
called "reserved powers" because of the reservation clause in the Constitution, which
says that powers not specifically given to the national government or prohibited to the
states are reserved for the states.