Wikipedia says: Systems engineering (or systems design engineering) as a field originated around the time of World War II. Large or highly complex engineering projects, such as the development of a new airliner or warship, are often decomposed into stages and managed throughout the entire life of the product or system.
This terminology also has the potential to be a confusing, mainly because the word system is so overloaded. A system can also be made up of many lower level systems (or sub-systems), still called systems.
So an Airliner is a system,
so is the Landing system (which is a part of the airliner) a system,
so is the bearing system (which is part of the landing system),
and so on; (finite).