“Two-phase flow is the simultaneous flow of two separate states of matter. The states can be any combination of gas, liquid, or solid, and can occur with or without simultaneous change from one state to another. Such change can occur with condensation, melting, sublimation, boiling, or the like. Since more than one phase can occur simultaneously, and since phase change can occur in the flow field, analysis of such flows is necessarily much more complex than when only one phase alone is flowing.”
Adapted from reference: Owen C. Jones, Elements of Two-Phase Flow, Boiling Heat Transfer: Modern Developments and Advances (1992), Elsevier Science Publishers B.V.