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.

On my Mac, there is a Help menu. Strange but true! In Finder Help, searching "symbol" I found "Symbols used in shortcuts". Your symbol is in there.

When used in this way, I would just call this symbol by the name "enter".

This is the ⌤ 'Enter Key' character, which is distinct from the similar ⌅ 'Projective' character.

⌤

Unicode Character 'UP ARROWHEAD BETWEEN TWO HORIZONTAL BARS' (U+2324)

Old name: 'ENTER KEY'

Wikipedia says:

The "Enter" key is commonly labelled with its name in plain text on generic PC keyboards, or with the symbol ⌤ (U+2324 up arrowhead between two horizontal bars) on many Apple Mac keyboards.