The immediate focus of interest which involves some input and output equal to input, overall is a system. Remaining all is surroundings.
In thermodynamics the system which is governed by a boundary, you will find resemblance the above statement.
So apply it to gaming which is a system of effects related to some initial condition. for example weapon fire, flash light, pickups, barriers, all these can be considered as independent systems for easier programming.
so in case of the systems of weapon that fires.
Keyboard input is the input, and output is bullet and flash, also another output is the sound of weapon fired. etc..
Luckily, in games the relation or sequence of these events of the system can be manipulated instead of governed by laws of physics as in case of real life systems.
So you can create a bullet that is metal but weapon fires with birds chirping ;)
if there is some code that manages these systems , it itself is also a system that consists sub-systems. hence to differentiate these kind of classes are called as manager class.
Basically your free to call it anything, for a farming game , it will be either farm manager or farm system, or bla bla