The acentric factor ω is a conceptual number introduced by Kenneth Pitzer in 1955, proven to be useful in the description of fluids. It has become a standard for the phase characterization of single & pure components, along with other state description parameters such as molecular weight, critical temperature, critical pressure, and critical volume (or critical compressibility). The acentric factor is also said to be a measure of the non-sphericity (centricity) of molecules.