Is Userland just another way to say User Space & User-Mode?
For example, would I be able to say:
There are various subsystems in Userland.
Instead of:
There are various subsystems in User-Mode.
..and would it be a valid way of saying that?
They're similar, but I think there's a subtle semantic difference. User-mode is a state of execution; usually it involves having restricted access to processor features (i.e. Ring 3 on x86), using syscalls to get services from the kernel, and not having kernel memory mapped. At any time a thread is either running in user mode or kernel mode.
Userland is the collection of software that runs in user-mode — for instance, you could say that Android has a Linux kernel but a radically modified userland, or that Debian GNU/kFreeBSD has a FreeBSD kernel but a Linux-derived userland. It's a colloquialism for everything that's not the kernel, or sometimes everything that would be considered the "operating system" except the kernel.