Final answer:
The powers necessary for the efficient operation of the U.S. government not explicitly listed in the Constitution are known as implied powers, stemming from the 'necessary and proper' or elastic clause in Article I, Section 8.
Step-by-step explanation:
The powers NOT stated in the United States Constitution but are necessary and proper to assure efficient operation of the country are called implied powers. These powers are not explicitly detailed in the Constitution;
however, they are understood to be necessary for the federal government to implement the enumerated powers that are listed. The constitutional basis for these implied powers comes from the "necessary and proper clause" found in Article I, Section 8 of the Constitution, also known as the elastic clause. This clause provides flexibility for the federal government to carry out its enumerated duties effectively.