ultra-violet radiation	A portion of the electromagnetic spectrum that is below visible light. Ultraviolet radiation has a shorter, or smaller, wavelength than visible light. Too much ultraviolet radiation can damage tissue (like human skin). Much of the ultraviolet radiation from the sun is absorbed within the atmospheric ozone layer before it reaches Earth's surface.
unicellular organism	An organism which consists of only one cell e.g., protozoans and certain algae.
upwelling	The process by which deep water masses rise, usually as a result of divergence and offshore currents. Upwelling water is usually colder and more nutrient-rich than the surface water that is replaced and results in higher productivity.
urbanisation	The high concentration of human beings in a limited space.