\documentclass{article}

\usepackage{nomencl}
\makenomenclature

\begin{document}

\section*{Main equations}

\begin{equation}
a=\frac{N}{A}
\end{equation}%

\nomenclature{$a$}{The number of angels per unit area}%
\nomenclature{$N$}{The number of angels per needle point}%
\nomenclature{$A$}{The area of the needle point}%

The equation $\sigma = m a$%
\nomenclature{$\sigma$}{The total mass of angels per unit area}%
\nomenclature{$m$}{The mass of one angel}
follows easily.

\printnomenclature

\end{document}