Behold, the squircle-oid!
This is the shape defined by the equation
(x/2)⁴ + y⁴ + z⁴ ≤ 1
I made it as part of a set of props for explaining Lᵖ norms. p=2 gives an ellipsoid, and in the limit p → ∞ you get a cuboid.
In reply to @christianp
it's available on thingiverse at thingiverse.com/thing:3866307
In reply to @christianp
... I mean inequality, not equation. Don't @ me