| Term | Definition |
| X Bracing | That form of bracing wherein a pair of diagonal braces cross near mid-length of the bracing members. |
| X-Axis | One of the axes in a three-dimensional system, that is not the y- or z-axis. |
| X-Ray | Electromagnetic radiations of a short wavelength that can penetrate various thicknesses of all solids. |
| X-Ray, Weld | See Weld X-Ray. |
| Xanthic | Containing yellow or pertaining to yellow color. |
| Xenon | A heavy colorless odorless inert gaseous element occurring in traces in the atmosphere and used in fluorescent lamps. |
| Xeric | Requiring only a small amount of moisture, as a xeric habitat. |
| Xerography | A dry electrostatic document copying process. |
| Xerothermic | A hot dry environment. |
| XO, XOO, XOOX | Designations of the arrangement of sliding and fixed panels in sliding glass doors and windows; X indicates a sliding panel; O indicates a fixed panel; see OX, OXO, OXXO. |
| Xylem | The portion of the tree trunk, branches, and roots that lies between the pith and the cambium. |
| Xylene | A solvent obtained from wood tar, coal tar, or petroleum. |
| Xylography | The art of making engravings on wood. |
| Xylol | Xylene. |
| Xylonite | A thermoplastic material like celluloid, used in model building. |
| Xyst | In Greek and Roman architecture, a long covered portico for exercise in bad weather. |