One part per 10**6 (million) of the SI unit of quantity of matter (the mole) per SI unit area per SI unit of time. This term is based on the number of photons in a certain waveband incident per unit time (s) on a unit area (m2) divided by the Avogadro constant (6.022 x 1023 mol-1). It is used commonly to describe PAR in the 400-700 nm waveband. Definition Source: Thimijan, Richard W., and Royal D. Heins. 1982. Photometric, Radiometric, and Quantum Light Units of Measure: A Review of Procedures for Interconversion. HortScience 18:818-822.