or number of moles 
of a fixed quantity of the substance to its volume 
pressure 
and temperature 
(and/or any other relevant thermodynamic coordinates). [P7.2]
for: any substance
is: an equation (usually an approximation) which relates the mass or number of moles of a fixed quantity of the substance to its volume pressure and temperature (and/or any other relevant thermodynamic coordinates). [P7.2]
is exemplified: by the equation of state of an ideal gas, 
[P7.2]
Copyright 1997, The Open University