| X-ALD | X-linked Adreno Leuko Dystrophy |
| XANES | X-ray Absorption Near Edge Spectroscopy |
| XFEL | X ray Free Electron Laser |
| XIAP | X-linked Inhibitor of Apoptosis Protein |
| XL | Cross Linking |
| XLMTM | X-Linked MyoTubular Myopathy |
| XLP | X-Linked Protoporphyria |
| XOI | xanthine oxidase inhibitor |
| XPS | X-ray Photoelectron Spectroscopy |
| XPS | eXtruded Poly Styrene |
| XR Reality | eXtended Reality |
| XR Release | eXtended Release |