What is a Quanta/quantum?

A Quantum (plural quanta) is a small packet of energy that shows the discrete (discontinuous / granular/corpuscular) nature of matter and light. In case of light,it is also called photon

As Einstein said:

        Energy E of a quantum is given by

                    E = hf = mc^2

where

             h = Planck's constant

              f = frequency of radiation 

            m = mass of matter

              c = speed of light