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. The most important parameter in deciding what line to use is its strength. This is the amount of weight the line can hold before snapping. One must balance the trade off between strength and visibility.
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, Dacron and Dyneema (UHMWPE). The most common type is monofilament, made of a single strand. There are also cofilament and fused lines. Braided and fused lines are now readily available.