A knot is a simple closed curve in 3-dimensional space.
What does that mean? Well, a loop like the one at the left is considered a knot in mathematical knot theory (it is a simple closed curve in 3-dimensional space). In fact this knot has a special name: The unknot. The unknot can be drawn with no crossings, and is also called a trivial knot. It is the simplest of all knots.
The Borromean knot
So called because the figure is found on the coat of arms of the Borromeo family, the Borromean Knot is a group of three rings which are linked in such a way that if any one of them is cut, all three become separated. In his late seminars, i.e., from Seminar XX onwards, Lacan uses the Borromean knot as a way of illustrating the interdependence of the three orders of the real, the symbolic and the imaginary. This corresponds to a rethinking of the relationship between language and the body in the structuring of the subject.