What is pure energy? Is it equal to radiant energy? What does that E in E=mc^2 stand for?
There is no such thing as "pure energy." Energy always appears in one form or another, e.g. light, heat, sound, movement.
To be exact, and as chroot said, there is no such thing as "pure energy." However people do use that term. When they do they usually are referring to the energy of matter which consists of particles which have zero rest mass. Although it sounds odd Einstein did state that EM radiation was to be considered to be "matter."
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