This International Standard deals with terms, letter symbols and units related to the flow of liquids through rotodynamic and positive displacement liquid pumps and associated installations. It serves as a means of clarifying communications between the installation designer, manufacturer, operator and plant constructor. This International Standard identifies the units in common usage but, all other legal units can be used. This International Standard deals solely with conditions described by positive values for the rate of flow and pump head. The definitions are set out showing first the most common form of a quantity followed by some frequently used variants. Other variants can be constructed and appropriate symbols evolved using the symbols and subscripts shown. Prefixes such as ""working"" and ""design"" can also be applied to the defined quantities. This International Standard is not concerned with terms, letter symbols and units referring to the component parts of rotodynamic and positive-displacement pumps and installations. Whenever possible, symbols and definitions conform to those used in ISO 31-0 and ISO 1000, with further explanations where these are deemed appropriate. Some deviations have been incorporated for reasons of consistency.