oblique mark vs separatrix

oblique mark

noun
  • Synonym of slash ⟨/⟩, sometimes (generally proscribed) inclusive of similar marks such as the backslash ⟨\⟩. 

separatrix

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

  • The line between regions having different magnetic fields. 

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

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

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