\documentclass[a4paper]{article}

\usepackage[style=long4colheaderborder]{glossaries}

\makeglossaries

\newglossaryentry{w}{name={$w$},
sort=w,
description={width},
symbol=m}

\newglossaryentry{M}{name={$M$},
sort=M,
description={mass},
symbol=kg}

\begin{document}

\printglossaries

The width, \gls{w}, is measured in meters. The mass, \gls{M} is
measured in kilograms.

\end{document}
