In science, a process that isn't reversible's called irreversible. This concept arises most frequently in thermodynamics, as applied to processes. Irreversibility's also used in economics to refer to investment or expenditures that involve large sunk costs. In thermodynamics, a change in the thermodynamic state of a system's irreversible if the system can't be restored to its former state by infinitesimal changes in some property of the system without expenditure of energy. An irreversible proce… (More on Irreversibility)