The EELS edge onset is the sudden rise in intensity preceding peaks, representing the ionization threshold which approximately corresponds to the inner-shell binding energy.

Inner shell excitations occur at energies:
         ΔE ≥ E_F - E_B ----------------------------------- (3437)
where,
         ΔE -- The energy loss
         E_F -- The Fermi level energy
         E_B -- Binding energy of the inner shell
The EELS feature corresponds to "core loss edges" or "edge onset" (ΔE = E_F - E_B).