Environment tabbing
LaTeX also knows a tabbing environment:
\= sets a tabulator, which can be again reached by \>. \kill cancels the line, but keeps the tabulator settings.
\documentclass{article}
\begin{document}
\begin{tabbing}
gamma\quad\= a Greek letter\kill
Kav \> average kinetic energy\\
N \> number of atoms\\
gamma \> a Greek letter
\end{tabbing}
\end{document}

Environment tabular
A tabular can be used:
\documentclass{article}
\begin{document}
\begin{tabular}{@{}ll@{}}
Kav & average kinetic energy\\
N & number of atoms\\
gamma & a Greek letter\\
\end{tabular}
\end{document}

Environment description
Such a list can also be set as description list, e.g.:
\documentclass{article}
\usepackage{enumitem}
\usepackage{calc}
\begin{document}
\begin{description}[labelwidth=\widthof{\bfseries gamma}]
\item[Kav] average kinetic energy
\item[N] number of atoms
\item[gamma] a Greek letter
\end{description}
\end{document}

\qquad? – CroCo Aug 29 '15 at 14:33tabis a fixed space, therefore\qquaddoes similar thing. For the exact output, of course extra work need to be done. – CroCo Aug 29 '15 at 14:37