My knowledge might be very out of date, but I remember a Linux process being an unit of execution as well as isolation. Creating a process without a thread is not possible afaik.

In contrast, Linux threads were implemented essentially as a hack - they were processes that shared memory and resources with their parent process, and were referred to internally as LWPs - lightweight processes.

I also remember a lot of Unix/Linux people not liking the idea of multithreading, preferring multiple processes to one, single-threaded process.