First recognized in 1900 by Max Planck, it was originally the proportionality constant between the minimal increment of energy, E, of a hypothetical electrically charged oscillator in a cavity that contained black body radiation, and the frequency, f, of its associated electromagnetic wave. In 1905 the value E, the minimal energy increment of a hypothetical oscillator, was theoretically associated by Einstein with a "quantum" or minimal element of the energy of the electromagnetic wave itself. The light quantum behaved in some respects as <answer>an electrically neutral particle</answer>, as opposed to an electromagnetic wave. It was eventually called the photon.
Instead of an electromagnetic wave, what did the light quantum behave as in some regards?
an electrically neutral particle