For an eclectic compilation of mathematical and scientific formulas and physical parameters pertinent to a variety of plasma regimes, ranging from laboratory devices to astrophysical objects, you may look into the mini-Bible of plasma physicists. To view the full text version you will need the Adobe Acrobat Reader that is available from Adobe for free. As mentioned above liquids and gases are what we call fluids. Fluids differ from solids in that they flow in response to an applied force. They tend to form faceted crystals if grown under the right conditions.
Solids have a nonzero shear modulus.
This shear modulus is a material property that tells us how a substance responds to shear forces, which change the shape of the substance without changing the volume. Finally solids have a fixed shape at low pressure and are usually hard to compress. Liquids have a fixed volume at low pressure and like the solids they are usually hard to compress. Gases on the other hand expand to occupy the available volume and are highly compressible. A solid tries to resist static shear distortions, while a fluid does not.