\documentclass{article}
\usepackage[cfg=sample05.cfg]{nomencl}
\makenomenclature
\begin{document}
\section*{Main equations}
\begin{equation}
  a=\frac{N}{A}
\end{equation}%
\newcommand{\magritte}{Not the number }
\nomenclature{$a$}{\magritte of angels per unit area}%
\nomenclature{$N$}{\magritte 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.

\renewcommand{\magritte}{The number }
\printnomenclature
\end{document}
