What is Emc square?

Answer 1

Correctly written it is E = mc^2

'E' is energy

'm' is mass

'c' is ther speed of light in a vacuum.

It is the Einsteinian theory relating energy to mass.

What it means is in a nuclear reactor , the mass of the nuclear fuel is converted into energy by the loss of mass. The mass placed in the reactor may be say 1 kg. It undergoes nuclear disintegration, forming two or more new elements of less mass ( say 0.999 kg) that the originsal mass . The loss being energy. !!!!! Notice it is a very tiny loss , but produces a huge amoint of energy.

Answer 2

Do you mean E = mc2 then if so it is Albert Einstein's famous relativity equation.