A thermodynamic cycle, operating as a heat engine or a heat pump, during which the working substance is in, or passes through, the vapor state. A vapor is a substance at or near its condensation point. It may be wet, dry, or slightly superheated. One hundred percent dryness is an exactly definable condition which is only transiently encountered in practice. Vapor behavior deviates so widely from the ideal gas laws that calculation requires the use of tables and graphs that give the experimentally determined properties of the fluid.