It's an object that could exist, except that ABED would not be a plane. It's a twisted plane. I don't know what the name of that shape is but you can take a p

It's an object that could exist, except that ABED would not be a plane. It's a twisted plane. I don't know what the name of that shape is but you can take a plane and twist it in a way that every point along a straight line between any two points on the surface is also on the surface, but yet the surface is not an actual plane. Edit: Go to wolfram alpha and graph graph y=(z-zx)/2 from x=-1 to x=3 and z=-1 to z=1 and you'll see an example.