separatrix

noun6

1. •
   A terminator: a line on a partially-illuminated surface separating the lit and shaded regions.
2. •
   The line between regions having different magnetic fields.
3. •
   The boundary separating two modes of behavior in a differential equation.
4. •
   (historical) The ⟨L⟩ or pipe ⟨|⟩ mark formerly used to divide integers from decimals.

   • Therefore in writing of decimall parts let the separatrix be always used. — 1660
5. •
   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.
6. •
   (no longer used) Synonym of decimal point, which replaced such marks.