Fishing line is any cord made for fishing. Important parameters of a fishing line are its length, material, and weight (thicker, sturdier lines are more visible to fish). The refractive index is also important - lines with a refractive index similar to water are almost invisible to fish.
Around the 1900s fishing lines were made from linen, silk and cotton. Modern fishing lines are almost entirely made from artificial substances, including nylon, polyethylene and Dacron. The commonest kind is monofilament, made of a single strand. There are also cofilament and fused lines.