a. Geom. [f. L. trop-us TROPE + -AL.] Pertaining to or constituting a trope: see TROPE 8.
1875. Cayley, Math. Papers, IX. 519. The quartic surface has also four tropes (planes which touch the surface along a conic)…. The conic of contact or tropal conic in each plan being the intersection of the plane with the before mentioned quadric surface. Ibid., 520. Ordinary tropal planes each touching the surface in a proper conic.
