Origin: Latin prefix sub-
Subsystem has 2 different meanings across 1 category:
a system that is part of some larger system
"The braking mechanism functions as a critical subsystem within the vehicle's overall safety system."
Alternative spelling of sub-system
"The technical manual lists both "subsystem" and "sub-system" as acceptable spellings for a component within a larger system."
In plain English: A subsystem is a smaller part of a larger system that works together to do a specific job.
"The car's braking system is just one subsystem that works together to keep you safe while driving."
The word subsystem is formed by combining the prefix sub- with system. It entered English as a straightforward compound meaning something that exists within or below another larger system.