cline

noun3

1. •
   A gradation in a character or phenotype within a species, deme, or other systematic group.
2. •
   Any graduated continuum.

   • This account effectively reconstructs the well-known grammaticalisation cline from anaphora to agreement, … — 2005
3. •
   A generalized circle.

   • Let C₁ and C₂ be two nonintersecting clines. Prove that there is a unique pair of points that are simultaneously symmetric to both C₁ and C₂. — 2001