In chemistry and physics, the exchange interaction is a quantum mechanical effect that only occurs between identical particles. Exchange energy is the energy released when two or more electrons with the same spin-exchange their positions in the degenerate orbitals of a subshell.
We can define it as:
The energy released when the anti-parallel electrons belonging to a degenerate (same energy) subshell are made to have parallel spins.
Exchange energy forms an important part of the covalent bond of many solids and is also responsible for ferromagnetic coupling.