Yes, it's a type of triangle, it's 2D, It has a hole (The Inside) And no, it's not an impossible figure. Unfortunate, isn't it?

(Just like us)

I see... It's an optical illusion just like 2D, 3D and 4D. No inside, no outside. When calls the achievement is an triangle.

That's no the SIDE

This actually has two dimensions. It's just a fake depth. And a fake non-integer dimension.