Quantum comes from the same root as “quantity” and is defined as a discrete packet of energy.
In school, we learn classical physics that works specifically with macroscopic objects. The main problem arose in the early 1900s when experiments such as ‘Compton scattering’ and the ‘photoelectric effect’ threw doubt upon the model of electromagnetic waves. Instead, EM radiation acts as a wave in experiments designed to show it as a wave and as a particle in experiments designed to show it as a particle. Enter quantum physics: EM radiation behaves as discrete packets (or “quanta”) of energy when the intention is to observe it this way. The duality of EM waves is where the field of quantum physics stands in opposition to classical physics.
