| X | Inductive reactance |
| XANES | X-ray Absorption Near-Edge Structure |
| XAS | X-ray Absorption Spectroscopy |
| XBRL | eXtensive Business Research Language |
| XC | Capacitive reactance |
| xHTML | eXtensible Hypertext Mark-up Language |
| XIP | eXecute In Place |
| XL | Inductive reactance |
| XLS | eXcimer Laser System; eXtended Light Scatterer |
| XMCL | eXtensible Media Common Language |
| XML | Extensible Markup Language |
| XML-RPC | XML Remote Procedure Calls |
| XMOD | Cross Modulation |
| XMP | X-ray Micro-Probe; eXtensible Metadata Platform |
| XOR | Logic term - XOR |
| XPD | X-ray Photoelectron Diffraction |
| XPRL | eXtensive Public Relations Language |
| XPS | X-ray Photoelectron Spectroscopy |
| XR | X-ray Reflectivity |
| XRD | X-ray Diffraction |
| XRF | X-ray fluorescence spectrometry |
| XRML | eXtensible Rights Markup Language |
| XSL | eXtensible Style sheet Language |
| XSLT | eXtensible Style sheet Language Transformations |
| XTEM | Cross sectional Transmission Electron Microscopy |
| Xe | Xenon |
| X-ray | Röntgen beam technology |
