What does it mean when a process is surrounded by parens in `ps`? I sometimes try to kill -9 Finder.app when it's misbehaving and I'm in a Terminal. This normally works but every once in awhile it leaves behind the pid, even when run with sudo, and the COMMAND portion of the output changes to (Finder).

Interestingly enough, this process only appears in the output of ps, not in Activity Monitor.app. No idea how to explain that.

At that point, I'm in for a reboot given my current knowledge, but I don't understand why. Can anyone explain what's happening and maybe how I can restart Finder without rebooting?

EDIT: I will note that once I'm in this state, I can usually attempt to go through the regular ⌘⌥⎋ method and Finder.app will be there. Once I've force quit it though, the name disappears forever from the Force Quit dialog and the zombie pid sticks around anyway.

I'll also note that Finder.app is treated specially in that dialog, where the button text turns from Force Quit to Relaunch, so perhaps that shows that OS X treats killing the Finder as special?
When you see a process in parentheses/brackets it means that ps could not determine its command (which appear in the CMD column). This might explain why you don't see it in the Activity Monitor.

You can restart Finder also outside a shell. Just click on the Apple symbol in the top left corner of the MacOS task bar, click on Force Quit and select Finder (usually at the bottom)

Generally, I find top more convenient than ps, but I think both should work equally fine.