Knot (fastener)
A knot was a means of fastening two cords together, or securing one cord around itself or another object.
It could also metaphorically refer to a difficult problem. Jean-Luc Picard enjoyed trying to prove Fermat's last theorem, because to him "it puts things in perspective," clarifying that, "in our arrogance, we feel we are so advanced, and yet we cannot unravel a simple knot tied by a part-time French mathematician, working alone, without a computer." (TNG: "The Royale")