1. With respect to radiation, a single point in space emitting radiation. The radiation from such a source may be expressed as flux per unit solid angle (W sr⁻¹). 2. In experimental studies of atmospheric turbulence and diffusion, a source of particulate matter from a single fixed point. There are continuous and instantaneous point sources. The analogous concepts of line source and plane source are also frequently encountered. 3. In hydrodynamics, a source of mass, that is, a singular point in the field where the equation of continuity fails.