What is "nephroid"

a. kidney-shaped n. (cx geometry English) A plane curve resembling two intersecting ellipses

The nephroid is a plane curve whose name means kidney-shaped (compare nephrology.) Although the term nephroid was used to describe other curves, it was applied to the curve in this article by Proctor in 1878. This and the information below may be verified in Lockwood, pp. 62–71 (see References).