Conchoid

noun

1. Any of a family of curves defined as the locus of points p, such that each p is on a line that passes through a given fixed point P and intersects a given curve, C, and the distance from p to the point of intersection with C is a specified constant (note that for nontrivial cases two such points p satisfy the criteria, and the resultant curve has two parts).

   "The Conchoid of Nicomedes is the conchoid of a straight line with respect to a point not on the line."

2. A conchoidal fracture in rock.