n.
Pronunciation: ' f ō - ˌ tän
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 \ f ō - ' tä-nik \ adjective