separatrix

Borrowed from Latin separatrix, the feminine form of separator (originally owing to an implied līnea (“line”)), from sēparāre (“to divide; to separate”) + -trix (forming female agents). First developed as a decimal mark among the medieval Arab mathematicians, whence a shorter variant gave rise to the decimal comma employed by many European countries and their former colonies.

separatrix (plural separatrixes or separatrices)

1. (typography, historical) The ⟨L⟩ or pipe ⟨|⟩ mark formerly used to divide integers from decimals.
   • 1660, Jonas Moore, Moor's Arithmetick, page 13:

     Therefore in writing of decimall parts let the separatrix be always used.

2. (typography, obsolete) Synonym of decimal point, which replaced such marks.
3. (typography) 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.
4. A terminator: a line on a partially-illuminated surface separating the lit and shaded regions.
5. (physics) The line between regions having different magnetic fields.
6. (mathematics) The boundary separating two modes of behavior in a differential equation.

• (marks): See pipe and slash
• (decimal point): See decimal point
• (line dividing areas of illumination): See terminator

• Merriam-Webster Online. "separatrix". Accessed 11 February 2016.
• Oxford English Dictionary, 1st ed. "separatrix, n." Oxford University Press (Oxford), 1912.