Biophotons (from the Greek βίος meaning "life" and φῶς meaning "light") are photons of light in the ultraviolet and low visible light range that are produced by a biological system.  They are non-thermal in origin, and the emission of biophotons is technically a type of bioluminescence, though  bioluminescence is  generally reserved for higher luminance luciferin/luciferase systems. The term biophoton used in this narrow sense should not be confused with the broader field of biophotonics, which studies the general interaction of light with biological systems.
Biological tissues typically produce an observed radiant emittance in the visible and ultraviolet frequencies ranging from 10 to 10 W/cm (approx 1-1000 photons/cm/second). This low level of light has a much weaker intensity than the visible light produced by bioluminescence, but biophotons are detectable above the background of thermal radiation that is emitted by tissues at their normal temperature.
While detection of biophotons has been reported by several groups, hypotheses that such biophotons indicate the state of biological tissues and facilitate a form of cellular communication are still under investigation,  Alexander Gurwitsch, who discovered the existence of biophotons, was awarded the Stalin Prize in 1941 for his work.