\begin{definition}[Definition:Bessel Function/Order]
Consider Bessel's equation:
:$x^2 \dfrac {\d^2 y} {\d x^2} + x \dfrac {\d y} {\d x} + \paren {x^2 - n^2} y = 0$
Let:
:$\map {J_n} x$ denote the Bessel function of the first kind
:$\map {Y_n} x$ denote the Bessel function of the second kind
be the solutions of Bessel's equation as defined.
The parameter $n$ is known as the '''order''' of the Bessel function.
\end{definition}