A curve
is osculating to
at
if it is tangent at
and has the same curvature
there. Osculating curves therefore satisfy
for ,
1, 2. The point of tangency is called a tacnode.
One of simplest examples of a pairs of osculating curves is and
, which osculate at the point
since for
, 1, 2,
is equal to 0, 0, and 2.