What is the Mac keyboard key with a symbol ⌅ of an up arrow pointing into a horizontal line?
It is called Projective, with Unicode U+2305, and it represents Enter which is different to Carriage Return, represented with a ↩. 

Although most of the time they work in the same way, there is a difference between Enter and Carriage Return. While Enter, in most cases, will make a GUI object operate at its default function, the Carriage Return is meant to reset the user's position to the beginning of the line of a text.

Just doing fn+Return should trigger it. 

On the short keyboards you'll either find the name or the icon.