Using the enumitem package:
\documentclass{article}
\usepackage{calc}
\usepackage{enumitem}
\newlength\widest
\begin{document}
red yellow blue, red yellow blue, red yellow blue,
red yellow blue, red yellow blue, red yellow blue,
\settowidth\widest{\textbf{word}}
\begin{description}[leftmargin=\dimexpr\widest+\labelsep\relax,labelindent=0pt,
labelwidth=\widest]
\item[word]
red yellow blue, red yellow blue, red yellow blue,
red yellow blue, red yellow blue, red yellow blue,
\item[that]
one two three four, one two three four, one two three four,
one two three four, one two three four, one two three four,
one two three four, one two three four, one two three four,
\end{description}
\end{document}

For an automatic calculation of the widest label, you can see my answer to Automatically set description list `labelwidth` based on widest label?.