Final answer:
The principle that mass can be converted to energy and vice versa, as described by E=mc², is known as mass-energy equivalence, a revolutionary concept from Einstein's theory of special relativity.
Step-by-step explanation:
The principle that mass and energy are not independent and can be converted from one to the other according to Einstein's formula E=mc² is mass-energy equivalence. This concept, derived from the theory of special relativity, has significant implications in the fields of physics and cosmology. It has been demonstrated in processes such as nuclear fission and nuclear fusion, where mass is converted into vast amounts of energy, as evident in the functioning of the Sun and nuclear power plants.