This term is defined in ECSS-E-ST-35C Rev. 1 as
See propulsion system (see 188.8.131.52).
This term is defined in ECSS-Q-ST-40C as
set of interdependent elements constituted to achieve a given objective by performing a specified function [IEC 50:1992]
This term is defined in ECSS-S-ST-00-01C as
set of interrelated or interacting functions constituted to achieve a specified objective