This is a system:
System has an Input, Output, State, Boundary, and Environment.
Systems are often layered on top each other. Layered systems are also called stacks.
MITM, interception at the boundary
Runtime system provides the environment in which other software is executed.
The runtime system is defined primarily by the operating system and by the language platform.
Runtime is a Layered System:
Software is supported built on top of its language
The language may be built on top multi-language platform runtime
The language platform runtime is built on top of the operating system
The operating system operates on top of the hardware system
All user space software operates on top of operating system kernel.