The Abstract Idea of Process

A process is a virtual machine, relying on system calls
It interacts trhough memory and system calls, nothing more

A process is a sequence of instructions
Running within a  virtual memory space
Communicating through file descriptors
Ready to handle signals
Carrying its own env, cwd, root, limits, ...

A thread, in Linux, is just a process
There's no technical reason to differentiate
The illusion of "threads" is built towards user space (but TID exists)
Sharing (or lack thereof) of state is handled by clone(2) bits

Every process is executing a binary file
There is no exception possible: no filesystem means no process
Kernel threads are an exception
Great idea, but out of scope now