ellipse

ellipse (the locus of a point such that the sum of the distances from that point to two other fixed points is constant; equivalently, the conic section that is the intersection of a cone with a plane that does not intersect the base of the cone); ellipsis (the removal from a phrase of a word which is grammatically needed, but which is clearly understood without having to be stated)

Learned borrowing from Latin ellīpsis, itself a borrowing from Ancient Greek ἔλλειψις (élleipsis).