A part of a process in which two fluids are in equilibrium. Such an equilibrium stage may also be referred to as a theoretical tray, equilibrium stage, or ideal stage. In a distillation column, a liquid and a vapour are close to being in equilibrium in the reboiler and partial condenser. On each of the trays within the column, the enrichment of vapour of the more volatile component is less than one theoretical stage. This means that more stages or trays are required in practice to achieve a desired separation. In absorption columns, the amount of a gas absorbed is a fraction of the amount absorbed in a theoretical stage. In a liquid–liquid solvent extraction process, a mixer-settler is close to one theoretical stage. See murphree plate efficiency.