I think it is 'enter.' You could change it if you wanted to. Go to: Edit>Keyboard Shortcuts...

Ehm -- no.

Shift + Return = Soft return, invokes the blight of forced justification upon your paragraphs.

It's the Enter key at the side of your numeric pad. It's called "Enter" to discern it from "Return", as there were days those keys were clearly marked that way.

By the way, if you don't like that key -- perhaps you are working on a laptop? --, you can always select your own, under Edit -> Keyboard Shortcuts -> Endless browsing throught the drop lists until you find "Column break". If you cannot locate it, you can click "Show Set". This will create a simple text edit file with all possible commands and its associated keystrokes, and there you can use search to find the category "Column Break" falls under.

You're welcome. :)