Definition:Knot (Knot Theory)

Definition
Given a submanifold $X \subset Y$ and an inclusion $i: X \to Y$ such that $i(X) = X$, a knotted embedding is an embedding $\phi: X \to Y$ (or the image of such an embedding) such that $\phi(X)$ is not homotopic to $i(X)$.

Sphere Knot
Frequently, the term knotted $n$-sphere is given to a knotted embedding $\phi:\mathbb S^n \to \R^{n+2}$.

Circle Knot
The description of the sphere is dropped for $\mathbb S^1$ and the term knot is used without qualification for knotted embeddings $\phi: \mathbb S^1 \to \R^3$.

Elementary Knot
Circle knots can often be quite wild and unwieldy - most of modern knot theory concerns itself with a specific kind of knot.

These knots are described as a finite set of points in $\R^3$ called $\left\{{ x_1, x_2, \dots, x_n }\right\}$, together with line segments from $x_i$ to $x_{i+1}$ and a line segment from $x_n$ to $x_1$.

The union of all these line segments is clearly a circle knot, or an unknot, an embedding of the circle which is homotopic to a circle.