Which mass is being referred to in E=mc^2?

[Aaron121]

Is the mass that appears in E=mc² the rest mass m₀, or the relativistic mass γm₀?

 

[Ibix]

Depends what you mean by $E$. Modern sources use $m$ to mean the rest mass, so $E=mc^2$ is the rest energy and $E=\gamma mc^2$ would be the total energy (rest plus kinetic). Older sources and pop sci sources could mean either.

 

[Orodruin]

Is the mass that appears in E=mc² the rest mass m₀, or the relativistic mass γm₀?

I would actually say neither. The big revelation of that equation (as originally conceived) is that the rest energy of an object is equal to the inertia at low velocities (multiplied by $c^2$). As such, $m$ would be the inertial mass from the limit of classical mechanics. This is now so ingrained into the nomenclature that we refer to the rest energy simply as "the mass" (modulo the multiplication by $c^2$, but we normally work in units where $c = 1$).

"Relativistic mass" is a historical leftover in popular literature, it should typically be avoided in scientific use.