ISA
It stands for International Standard Atmosphere and is an Internationally accepted model of how the Pressure, Temperature, Density and Viscocity of the Atmoshphere changes with Altitude.
At Sea Level the model is based on an Air Temperature of +15 Deg C, a QNH of 1013.2Mb and a Temperature Lapse rate of -1.98 Deg C per 1000ft.
Richard