separatrix

Meanings

1. The ⟨L⟩ or pipe ⟨|⟩ mark formerly used to divide integers from decimals.
2. Synonym of decimal point, which replaced such marks.
3. 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. The line between regions having different magnetic fields.
6. The boundary separating two modes of behavior in a differential equation.

Etymology

Learned borrowing from Latin sēparātrīx, the feminine form of separator (originally owing to an implied līnea (“line”)), from sēparāre (“to divide; to separate”) + -trix (forming female agents). By surface analysis, separate + -trix. 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.