# What's the name of this parameter?

$$P_m=P_n*{G\frac1{G_{STC}}}*[1-\gamma*(T_c-25)]$$

This equation is applied on sollar cells. $$P_m$$ is the mean power produced in 15 minutes, $$P_n$$ is the nominal power [W], G is the insolation [W/m2], $$G_{STC}$$ (the subscript still in the wrong place, I don't know why) is the radiation [W/m2] in standard testing conditions reference here. Last, but nost least, $$T_c$$ is the cell temperature.

What is the parameter gamma ($$\gamma$$)? I have a value for gamma, 0.005 [adimensional], but I don't know what it means or its definition.