Who discovered 'X-rays' first

W. C. Rontgen discovered X-rays first.
W. C. Rontgen reported the discovery of X-rays in December 1895 after seven weeks of assiduous work during which he had studied the properties of this new type of radiation able to go through screens of notable thickness. He named them X-rays to underline the fact that their nature was unknown.
X-radiation is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3 × 10¹⁶ Hz to 3 × 10¹⁹ Hz) and energies in the range 145 eV to 124 keV.