The photostationary state of a reversible photochemical reaction is the equilibrium chemical composition under a specific kind of electromagnetic irradiation (usually a single wavelength of visible or UV radiation).
It is a property of particular importance in photochromic compounds, often used as a measure of their practical efficiency and usually quoted as a ratio or percentage.
The position of the photostationary state is primarily a function of the irradiation parameters, the absorbance spectra of the chemical species, and the quantum yields of the reactions.  The photostationary state can be very different from the composition of a mixture at thermodynamic equilibrium.  As a consequence, photochemistry can be used to produce compositions that are "contra-thermodynamic".
For instance, although cis-stilbene is "uphill" from trans-stilbene in a thermodynamic sense, irradiation of trans-stilbene results in a mixture that is predominantly the cis isomer.  As an extreme example, irradiation of benzene at 237 to 254 nm results in formation of benzvalene, an isomer of benzene that is 71 kcal/mol higher in energy than benzene itself.