What are some common ways to add variable descriptions with equations? [closed]

Question

I'm looking for a good way to add variable descriptions immediately following equations. In plain text, it might look something like this:

r_p = a(1-e)

where a is semi-major axis

(space) e is eccentricity (perhaps e is neatly aligned with a)

Further, I'm interested in placing this equation and the descriptions inside of a list:

\begin{itemize}
\item Periapse
    \begin{itemize}
    \item The point of closest approach to the foci
    \[r_p = a(1-e)\]
    (variable descriptions)
    \end{itemize}
\item Apoapse
    \begin{...
      ...
\end{itemize}

Answer 1

Like this?

% arara: pdflatex

\documentclass{article}
\usepackage{array}
\newcommand{\where}[1]{where~\begin{tabular}[t]{@{}>{$}c<{$}@{ }l@{}}#1\end{tabular}}

\begin{document}
\begin{itemize}
    \item Periapse
    \begin{itemize}
        \item The point of closest approach to the foci
        \[r_p = a(1-e)\]
        \where{a & is semi-major axis \\e & is eccentricity}
    \end{itemize}
    \item Apoapse
\end{itemize}
\end{document}