Any portion of the universe that can be isolated completely and arbitrarily from the rest for consideration of the changes that may occur within it under varied conditions. In a closed system, energy may cross the system boundary, but matter may not. In an open system, both energy and matter may enter or leave as required. An equilibrium system is closed with all phases in their lowest energy states. The variance (degrees of freedom) of an equilibrium system is its number of components minus its number of phases plus two. A steady-state system is open with all phases in their lowest energy states while matter streams through it. Systems may be described by the number of their components; 2308 e.g., unary for one component, binary for two, ternary for three, etc. They are commonly defined in terms of their components; e.g., the system CaO-MgO-SiO₂-H₂O is a quaternary system.