Which is why I took the effort to explicitly refer to Windows NT on my comment, already expecting some traditional answers from UNIX folks.
Also due to historical reasons POSIX threads are the outcome of every UNIX going their own way implementing threads, finally coming to an agreement years later, with all the plus and minus of relying in POSIX for portable code.
How are those not simply child processes? I don't understand your use of the word 'threads' here.
Does the Unix world not distinguish between threads and processes? In Win32, threads exist within processes, and you can create new threads or child processes.
Second answer: Linux doesn't differentiate between threads and processes. It has a "thread group ID" that serves a small number of purposes, and the rest of the difference is just whether the threads happen to share the same address space.
The unit of execution is the thread.
On the UNIX world it depends on which UNIX you are talking about.
Linux has a similar model to Windows NT nowadays, hence clone() as key primitive.
Other UNIXes have different approaches.