diagonal vs separatrix

diagonal

noun
  • A line or cut across a fabric at an oblique angle to its sides. 

  • Synonym of slash ⟨/⟩. 

  • A line joining non-adjacent vertices of a polygon. 

  • A line or plane at an oblique angle to another. 

adj
  • Having a slanted or oblique direction. 

  • Of or related to the cater-corner (diagonally opposite) legs of a quadruped, whether the front left and back right or front right and back left. 

  • Joining two nonadjacent vertices (of a polygon or polyhedron). 

  • Having slanted or oblique lines or markings. 

separatrix

noun
  • A terminator: a line on a partially-illuminated surface separating the lit and shaded regions. 

  • The ⟨L⟩ or pipe ⟨|⟩ mark formerly used to divide integers from decimals. 

  • The proofreader's mark resembling a slash ⟨ / ⟩ or vertical bar ⟨ | ⟩ placed after a note in the margin to indicate that it should replace the item(s) struckthrough in the running text or to separate it from other margin notes. 

  • The line between regions having different magnetic fields. 

  • The boundary separating two modes of behavior in a differential equation. 

How often have the words diagonal and separatrix occurred in a corpus of books? (source: Google Ngram Viewer )