The exact accounting of all material that enters, leaves, accumulates, or is depleted within a given period of operation across an imaginary boundary of a process or part of a process. It is in effect an expression of the law of conservation of mass in accounting terms. Where the conditions of a process are steady and unvarying with time, the material input flow across the boundary equals the output flow. Depending on whether chemical or physical change occurs, material balances may be conveniently calculated and expressed in either mass or molar terms.