An equation of state is a formula describing the interconnection between various macroscopically measurable properties of a system.
In physics and thermodynamics, an equation of state is a relation between state variables. More specifically, an equation of state is a thermodynamic equation describing the state of matter under a given set of physical conditions. It is a constitutive equation which provides a mathematical relationship between two or more state functions associated with the matter, such as its temperature, pressure, volume, or internal energy. Equations of state are useful in describing the properties of fluids, mixtures of fluids, solids, and even the interior of stars.
http://en.wikipedia.org/wiki/Equation_of_state
van der Waals equation
http://en.wikipedia.org/wiki/Van_der_Waals_equation
STATISTICAL PHYSICS I
Instructor: D. E. Sullivan
http://www.physics.uoguelph.ca/phys7040/phys7040.html