The line separating the numerator from the denominator, whether horizontal, (sometimes proscribed) oblique, or diagonal.
The ⟨L⟩ or pipe ⟨|⟩ mark formerly used to divide integers from decimals.
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.
The line between regions having different magnetic fields.
The boundary separating two modes of behavior in a differential equation.
A terminator: a line on a partially-illuminated surface separating the lit and shaded regions.