align list of symbols with dash

The KOMA-Script classes have a list that basically does what you want:

\begin{labeling}[<separator>]{<widest label>}
  ...
\end{labeling}

This evironment can be used with the standard classes, too, by loading the scrextend package. The list would then maybe look like:

\begin{labeling}[---]{\hspace*{4em}}
  \item[$P$] pressure 
  \item[$\eta$] viscosity
  \item[$\lambda(P,T)$] thermal conductivity
\end{labeling}

You can probably make nomencl use the list and you can definitely customize acro or glossaries to use it. Here's a way with acro:

\documentclass{article}
\usepackage{scrextend}% only if you don't use a KOMA-Script class
\usepackage{acro}

\newenvironment{myacrolist}
  {\labeling[---]{\hspace*{4em}}}% choose length for widest label
  {\endlabeling}
\acsetup{list-type=myacrolist}

\DeclareAcronym{pressure}{
  short = \ensuremath{P} ,
  long  = pressure ,
  sort  = pressure
}
\DeclareAcronym{viscosity}{
  short = \ensuremath{\eta} ,
  long  = viscosity ,
  sort  = viscosity
}
\DeclareAcronym{therm-conduct}{
  short = \ensuremath{\lambda(P,T)} ,
  long  = thermal conductivity ,
  sort  = conductivity thermal
}
\begin{document}

\acuseall
\printacronyms[name=Nomenclature]

\end{document}

There are many ways, the following uses a simple tabular (for more than one page, package longtable can be used):

\documentclass{article}
\usepackage{array}

\begin{document}
\begin{tabular}{@{}>{$}l<{$}@{ --- }l@{}}
 p & pressure \\
 \eta & viscosity \\
 \lambda(P,T) & thermal conductivity \\
\end{tabular}
\end{document}

Remarks:

• The dash symbol is added in the tabular's preamble between the first and second column.
• Math mode is enabled for the first column with the help of package array and its specification with > and >.

You can capture this using table. See below:

\begin{tabular}{llp{.7\textwidth}}
$P$ &---& pressure\\
$\eta$ &---& viscosity\\
$\lambda(P,T)$ &---& thermal conductivity\\
\end{tabular}

Note: I have edited the answer to include p{<width>} for long sentences.