Main Entry: pho·ton

Pronunciation: \ˈfō-ˌtän\

Function: noun

Etymology: phot- + ²-on

Date: 1916

1 : a unit of intensity of light at the retina equal to the illumination received per square millimeter of a pupillary area from a surface having a brightness of one candle per square meter
2 : a quantum of electromagnetic radiation

— pho·ton·ic \fō-ˈtä-nik\ adjective