This International Standard defines terms applying to active control units of ships. The main aim of this International Standard is to give clear enough definitions for each term to be understood identically by all specialists concerned. This International Standard applies to active control units used as auxiliary propellers to improve manoeuvrability of a vessel, to provide directional stability or positioning of a vessel. The units covered may, however, also be used as main propellers. The terms defined in this International Standard shall be used in documents of all kinds.