Every system is made up of atleast the following components:
- Input
- States
- Initial State
- Transitions
- Output
Machine is also a system. Systems make transition from one state to another based on the input. The result of such transitions is the output.
You feed something to machine to get something desired out of it. What you feed is input and what you expect is output!
Mathematically, Input is the domain and output is the co-domain.