Einstein's famous equation, E = mc^2, relates energy (E) to mass (m) and the speed of light (c). It states that the energy of an object at rest is equal to its mass times the square of the speed of light.
In this equation, mc^2 represents the rest energy of an object, which is the energy it possesses even when it is not in motion. This term specifically refers to the energy associated with an object's mass. Therefore, mc^2 can be seen as a property of energy, specifically the energy content equivalent to an object's mass.
However, it's important to note that energy is not limited to just the rest energy of an object. There are various forms of energy, including kinetic energy (associated with motion), potential energy (associated with position or stored energy), thermal energy, electromagnetic energy, and many others. So, while mc^2 is a property of energy, it does not fully encompass the entire range of energy phenomena.