The definitions on this page originally appeared on the United States Environmental Protection Agency's website.

Zero Air: Atmospheric air purified to contain less than 0.1 ppm total hydrocarbons.

Zero-impact: having no environmental footprint, as in products designed to create no emissions and leave no waste; also seen as zero-carbon impact, refering specifically to creating no CO2 emissions.

Zooplankton: Small (often microscopic) free-floating aquatic plants or animals.

'Zone of Saturation': The layer beneath the surface of the land containing openings that may fill with water.