separatrix

(typography, historical) The ⟨L⟩ or pipe ⟨|⟩ mark formerly used to divide integers from decimals.

(typography, obsolete) Synonym of decimal point, which replaced such marks.

(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.

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

(physics) The line between regions having different magnetic fields.

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