Math. [f. L. tactu-s touch + INVARIANT.] (See quots.)
1856. Cayley, Math. Papers, II. 320. The function which, equated to zero, expresses the result of the elimination is an in variant which (from its geometrical signification) might be termed the Tactinvariant of the two quantics.
1873. Salmon, Higher Plane Curves, iii. (1879), 80. The condition that two curves U, V, should touch (which condition is called their tact-invariant).
