Equations and defining equation variables

As detailed in align vs equation there are some occasions when the equation environment is more appropriate

I have moved your \text{...} into text mode in between the equation and align environment.

% arara: pdflatex
% !arara: indent: {overwrite: true, trace: true}
\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{equation}\label{eq:2.1}
    \frac{J_1(a k s)}{\sqrt{\pi } s}
\end{equation}
where $a$ is lens radius
\begin{align*}
      & k=\frac{2 \pi }{\lambda }                                                                      \\
      & s=\sqrt{\left(\frac{u}{z_2}+\frac{x}{z_1}\right)^2+\left(\frac{v}{z_2}+\frac{y}{z_1}\right)^2} 
\end{align*}                

or else (following the comment)

where 
\begin{align*}
      & a\text{ is lens radius}                                                                        \\
      & k=\frac{2 \pi }{\lambda }                                                                      \\
      & s=\sqrt{\left(\frac{u}{z_2}+\frac{x}{z_1}\right)^2+\left(\frac{v}{z_2}+\frac{y}{z_1}\right)^2} 
\end{align*}                

\end{document}

Option 1:

\documentclass[preview,border=12pt]{standalone}
\usepackage{amsmath}

\begin{document}
\begin{gather}\label{eq:2.1}
\frac{J_1(a k s)}{\sqrt{\pi } s}\\
\intertext{where $a$ is lens radius}
\begin{aligned}
k &=\frac{2 \pi }{\lambda }\\
s &=\sqrt{\left(\frac{u}{z_2}+\frac{x}{z_1}\right){}^2+\left(\frac{v}{z_2}+\frac{y}{z_1}\right){}^2}
\end{aligned}\notag
\end{gather}
\end{document}

Option 2:

\documentclass[preview,border=12pt]{standalone}
\usepackage{amsmath}

\begin{document}
\begin{gather}\label{eq:2.1}
\frac{J_1(a k s)}{\sqrt{\pi } s}\\
\begin{aligned}
\text{where } &a \text{ is lens radius}\\
&k =\frac{2 \pi }{\lambda }\\
&s =\sqrt{\left(\frac{u}{z_2}+\frac{x}{z_1}\right){}^2+\left(\frac{v}{z_2}+\frac{y}{z_1}\right){}^2}
\end{aligned}\notag
\end{gather}
\end{document}

Option 3:

\documentclass[preview,border=12pt]{standalone}
\usepackage{amsmath}

\begin{document}
\begin{gather}\label{eq:2.1}
\frac{J_1(a k s)}{\sqrt{\pi } s}\\
\intertext{where}
\begin{aligned}
&a \text{ is lens radius}\\
&k =\frac{2 \pi }{\lambda }\\
&s =\sqrt{\left(\frac{u}{z_2}+\frac{x}{z_1}\right){}^2+\left(\frac{v}{z_2}+\frac{y}{z_1}\right){}^2}
\end{aligned}\notag
\end{gather}
\end{document}

Option 4:

Using \shortintertext (Peter Grill's idea):

\documentclass[preview,border=12pt]{standalone}
\usepackage{mathtools}

\begin{document}

\begin{gather}\label{eq:2.1}
\frac{J_1(a k s)}{\sqrt{\pi } s}\\
\shortintertext{where}
\begin{aligned}
&a \text{ is lens radius}\\
&k =\frac{2 \pi }{\lambda }\\
&s =\sqrt{\left(\frac{u}{z_2}+\frac{x}{z_1}\right){}^2+\left(\frac{v}{z_2}+\frac{y}{z_1}\right){}^2}
\end{aligned}\notag
\end{gather}

\end{document}

The difference between \intertext and \shortintertext: