A crunode, also known as an ordinary double point, of a plane curve is point where a curve intersects itself so that two branches of the curve have distinct tangent lines. The Maclaurin trisectrix, shown above, has a crunode at the origin.
Crunode
See also
Acnode, Ordinary Double Point, Spinode, TacnodeExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Crunode." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Crunode.html