Reserved Power Law & Legal Definition. Reserved power is a political power that is not enumerated or prohibited by a constitution, but instead is reserved by the constitution for a specified political authority, such as a state government.Tenth Amendment of the U.S. Constitution provides for reserve power.eserved Power Law & Legal Definition. Reserved power is a political power that is not enumerated or prohibited by a constitution, but instead is reserved by the constitution for a specified political authority, such as a state government.