Description list - How to put the definition on a new line?

You can do that with a \hfill. No extra packages or multiple lines of code are needed :)

\documentclass{article}
\begin{document}
\begin{description}
  \item[First] \hfill \\ The first item
  \item[Second] \hfill \\ The second item
  \item[Third] \hfill \\ The third etc \ldots
\end{description}
\end{document}

Will Robertson's answer can affect all the description environments in the entire document. To apply the desired setting to a particular description, you may use [style=nextline] appended to \begin{description}

\usepackage{enumitem}

\begin{description}[style=nextline]
   \item[foo] bar
   \item[baz] bang
\end{description}

This will apply your style only to that particular description. Other descriptions will work with default behavior.

If you are using enumitem, the correct way to set the default style is like this:

\usepackage{enumitem}
\setlist[description]{style=nextline}