Squircle

Squircle centred on the origin (a = b = 0) with minor radius r = 1: x4 + y4 = 1

A squircle is a shape intermediate between a square and a circle. There are at least two definitions of "squircle" in use, the most common of which is based on the superellipse. The word "squircle" is a portmanteau of the words "square" and "circle". Squircles have been applied in design and optics.