A concentrated source (especially of radiation or pollution) that is spatially constricted WordNet 3.0 & Open English WordNet

A single identifiable localized source of something. OmegaWiki

A source of pollution, radiation, waves, fluid or other substance that has one specific location (and negligible physical extent, distinguishing a point source from other source geometries). In simulations they can often be approximated using mathematical point sources. Wiktionary