An oval whose construction is illustrated in the above diagram.
Moss's Egg
See also
Circle-Circle Intersection, Egg, Lens, OvalExplore with Wolfram|Alpha
References
Dixon, R. Mathographics. New York: Dover, p. 5, 1991.Referenced on Wolfram|Alpha
Moss's EggCite this as:
Weisstein, Eric W. "Moss's Egg." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/MosssEgg.html