A knot can be defined as a non-self-intersecting broken line whose endpoints coincide: when such a knot is constrained to lie in a plane, then it is simply a polygon. Wiktionary

A knot in its original sense can be modeled as a mathematical knot (or link) as follows: if the knot is made with a single piece of rope, then abstract the shape of that rope and then extend the working end to merge it with the standing end, yielding a mathematical knot. If the knot is attached to a metal ring, then that metal ring can be modeled as a trivial knot and the pair of knots become a link. If more than one mathematical knot (or link) can be thus obtained, then the simplest one (avoiding detours) is probably the one which one would want. Wiktionary