You mention VMS. Surely you've heard of lib$spawn(), not to mention the DCL command spawn, which is the natural way of creating a subprocess to do something different from the parent (as distinct from launching a detached process, which is roughly the equivalent of a nohup). The posix layer implements fork, but this is not the native VMS way of doing or thinking about things.

Digital had a lot to do with the invention of threads. PThreads are a derivative of DecThreads, and VMS was one of the platforms on which they were originally targetted. The reason for this was that there is a substantial overhead in process creation - spawns were expensive, and threads provided a convenient way of multitasking inside a process's address space.

Admittedly, my knowledge of VMS is less than my knowledge of most of those other systems, and I am pretty much an end user on that platform (inasmuch as there is any such thing as an end user on VMS). I really should turn on my Vax more often and play around with it more.

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Sanity? Oh, yeah, I've got all kinds of sanity. In fact, I've developed whole new kinds of sanity. You can just call me "Mister Sanity". Why, I've got so much sanity it's driving me crazy.

Yet, I've never heard of spawn in this context before. And I can't seem to find it listed on Wikipedia, either. What type of system is it associated with, and what are the semantic differences between spawn versus fork or threads?

"To spawn" is to produce offspring, hence, creating a child process. No mystery there. You can do it by calling system, for example.