Monofilament fishing line

Monofilament fishing line (shortened to just mono) is fishing line made from a single fiber of plastic material, as opposed to multifilament or braided fishing lines constructed from multiple strands of fibers. Most fishing lines are now nylon monofilament because they are cheap to manufacture and can be produced in a range of diameters which have different tensile strengths (called "tests" after the process of tensile testing, or possibly as a portmanteau of "tensile" and "strength").

Source: Wikipedia — Monofilament fishing line (CC BY-SA 4.0)