How can we define energy?

Question

Can we call matter without mass as energy? just a simple defintion to the word energy; massless matter= energy? and I'm not asking about photons.

Answer 1

Both in classical as in quantum mechanics, energy is the conserved charge (in the sense of the Noether's theorem) of a system (Hamiltonian) which is invariant up to time translations.

This is the most general definition of energy. It does not depend on the nature of the system (being either matter or radiation). Also note that at least in special relativity, mass and energy are basically the same thing (in the rest frame).

Noether's theorem and conservation of energy