The physicist John Archibald Wheeler, in a Princeton University classroom in 1967, argued with Einstein about quantum theory.
Different physicists have different answers.
It's a bundle of energy, an indivisible unit that can be sliced no more. Max Planck showed us a hundred years ago that light is emitted not in a smooth, steady flow, but in quanta. Then physicists found quantum jumps of energy, the quantum of electric charge and more. In the small-scale world, everything is lumpy. And more than just lumpy.