The whitish vertical lines in laid paper that are produced by heavier wires, or "chains," that support the wire mesh in the paper mold.
In common usage the term may be spelled either with ("chain-lines") or without the hyphen.
سلك السلسلة; خيط مسلسل
Ligne de chaînette; Pontuseau
Raymond Clemens and Timothy Graham, Introduction to Manuscript Studies (Ithaca, NY: Cornell University Press, 2007), 6-9.