Geom. [f. L. tac-tus touch + NODE.] A point at which two parts of the same curve have ordinary contact.
1852. Cayley, Math. Papers (1889), II. 28. The tacnode is a double point where two branches touch.
1873. Salmon, Higher Plane Curves, 207. Two nodes may coincide, giving rise to the singularity called a tacnode; this is in fact an ordinary (two-pointed) contact of two branches of the curve.
attrib. tacnode-cusp, the singularity of a curve which arises when a cusp and an immediately following tangency of the two branches coalesce.
1873. Salmon, Higher Plane Curves (1879), 207.
