separatrix

Etymology
Borrowed from, the feminine form of (originally owing to an implied ), from  + ; equivalent to. First developed as a decimal mark among the medieval Arab mathematicians, whence a shorter variant gave rise to the employed by many European countries and their former colonies.

Noun

 * 1)  The ⟨L⟩ or pipe ⟨|⟩ mark formerly used to divide integers from decimals.
 * , which replaced such marks.
 * 1)  The proofreader's mark resembling a slash ⟨&thinsp;/&thinsp;⟩ or vertical bar ⟨&thinsp;''' 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.
 * 2) A terminator: a line on a partially-illuminated surface separating the lit and shaded regions.
 * 3)  The line between regions having different magnetic fields.
 * 4)  The boundary separating two modes of behavior in a differential equation.
 * 1)  The line between regions having different magnetic fields.
 * 2)  The boundary separating two modes of behavior in a differential equation.