In thermodynamics, mass-energy equivalence is a principle which states that mass of matter and energy are interconvertable.

History
The first to state that matter and energy might be interconvertable was English physicist Isaac Newton who in his 1717 “Query 30” of his Opticks states:

“Are not the gross bodies and light convertible into one another, and may not bodies receive much of their activity from the particles of light which enter their composition?”

In 1905, German born American physicist Albert Einstein stated:

“If a body gives off the energy L in the form of radiation, its mass diminishes by L/c² .”

which implies that mass can be converted into energy according to the following equality:

$\Delta m = \frac{L}{c^2} \,$

Or in modern notation:

$E = mc^2 \,\!$

Particle physics
In modern particle physics terms, the matter portion of the universe is defined as "fermions" and the energy portion of the universe is defined as "bosons", whereby extrapolation, the matter energy equivalence principle states that fermions and bosons are different manifestations of the same thing and or interconvertable.