The m is for mass and c is for the speed of light. The attached link is Albert Einstein explaining the theory of relativity. It has subtitles as his accent is very thick.
The mass--energy equivalence is often written E=MC2 .
E is energy, M is mass and C is the speed of light in a vacuum.
Mass--energy equivalence was proposed in Albert Einstein's 1905 paper, "Does...
If you're talking about the mass/energy equivalence that arises out of Einstein's Relativity, it's been verified in thousands of experiments over the past 70 years. No serious question remains, and...