The term photon energy is used for the energy carried by a single photon of electromagnetic radiation (EMR). It is directly related to the wave's frequency:
E = h × f
A typical unit of measurement is the electron volt (eV). For short-wavelength, high-frequency EMR (gamma rays, X-rays, and often the shorter-wavelength portion of ultraviolet), it is often used to describe the EMR, whereas wavelength or frequency is more often used for longer-wavelength EMR.