What is the meaning of Separatrix?

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

Synonym of decimal point, which replaced such marks.

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.

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

The line between regions having different magnetic fields.

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

Source: wiktionary.org