An ideal solution is any solution which obeys Raoult's law.
Raoult's law gives the relationship between a solution's vapor pressure and a solvent's vapor pressure when the solute has no measurable vapor pressure (is nonvolatile).
The law is stated as follows: "The partial pressure of a solvent over a solution, P1, is given by the vapor pressure of the pure solvent, P°1, times the mole fraction of the solvent in the solution, X1," or P1 = X1*P°1
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.