| Acronym | Meaning |
| WTQA | Waste Testing And Quality Assurance |
| WTRXLPE | Water-tree Retardant Crosslinked Polyethylene [elec. Insulation] |
| WURST | Wideband |
| WVTR | Water Vapor Transmission Rate |
| WWAP | World Water Assessment Programme |
| WWCOT | Whisker Wall Coated Open Tubular Column |
| WWEMA | Waste And Wastewater Equipment Manufacturers Association |
| WWM | Wigner-Weyl-Moyal (formalism In Quantum Mechanics) |
| WWPLOT | Whisker Wall Porous Layer Open Tubular Column |
| WWSCOT | Whisker Wall Support Coated Open Tubular Column |
| WWT | Wastewater Treatment |
| WWTP | Wastewater Treatment Plant Or Process |
| WWW | World Wide Web |
| X-SAR | X-Band Synthetic Aperture Radar |
| XAES | X-ray Excited Auger Electron Spectroscopy |
| XAFS | X-ray Absorption Fine Structure |
| XANES | X-ray Absorption Near Edge Structure |
| XAPS | X-ray Appearance Potential Spectroscopy |
| XAS | X-ray Absorption Spectroscopy |
| XBIS | X-ray Bremsstrahlung Isochromat Spectroscopy |
| XC | Exchange-coulomb Potential |
| XCC | Expectation-value Coupled-cluster (in Correlation Energies And Electronic Properties Of Many-electron Systems Calcns.) |
| XD | Exothermic Dispersion |
| XDBP | Halogenated Disinfection By-product |
| XEAES | X-ray Excited Auger Electron Spectroscopy |
| XEAPS | X-ray Excited Electron Appearance Potential Spectroscopy |
| XEDS | X-ray Energy Dispersive Spectroscopy |
| XEOL | X-ray Excited Optical Luminescence |
| XES | X-ray Emission Spectroscopy |
| XFA | X-ray Fluorescence Analysis |
| XFH | X-ray Fluorescence Holography |
| XGRS | X-ray/gamma-ray Spectrometer |
| XHDPE | Cross-linked |
| XHV | Extremely High Vacuum |
| XIAES | X-ray Induced Auger Emission Spectroscopy |
| XIS | X-ray Isochromat Spectroscopy |
| XLPE | Crosslinked Polyethylene |
| XMA | X-ray Microanalysis |
| XMCD | X-ray Magnetic Circular Dichroism |
| XMLD | X-ray Magnetic Linear Dichroism |
| XMM | X-ray Multi-Mirror |
| XMP | Xanthosine 5'-phosphate |
| XMPA | X-ray Electron Microprobe Analysis |
| XNBR | Carboxylated Nitrile Rubber |
| XPD | X-ray Photoelectron Diffraction |
| XPED | X-ray Photoelectron Diffractometry |
| XPEEM | X-ray Photoemission Electron Microscopy |
| XPES | X-ray Photoelectron Spectroscopy |
| XPM | Cross-phase Modulation |
| XPM | X-ray Photoelectron Microscopy |