subsystem       
noun [ C ] (also sub-system) uk/ˈsʌbˌsɪs.təm/ us/ˈsʌbˌsɪs.təm/
 子系统
a system that forms part of a larger system
  These individual languages can be considered as subsystems of one linguistic system.
 The human body is a system which is comprised of a number of sub-systems. 人体是一个由若干个子系统组成的系统。 
 - More examples
 - The mechanism is composed of four main subsystems.
 - This subsystem contains a collection of tools that perform various functions.
 - Rainforests are composed of many different subsystems .
 - Some of the electricity that the power plant creates is used to operate the subsystems of the power plant itself.