vergency

noun2

1. •
   (old-fashioned) The act of verging or approaching; tendency, inclination towards something.

   • the Earth is moved about the Sun by its Inclination and Vergency towards it — 1704
2. •
   The reciprocal of the focal distance of a lens, used as a measure of the divergence or convergence of a pencil of rays.

   • When, by the pencil becoming oblique to the surface, the vergency produced on the pencil becomes changed, the primary and secondary focal points, V and H, separate […] — 1863