答え1
@David Carlisle の提案どおり、'title' が 1 行に 1 つだけある説明環境を求めているようです。これは次のようにすると簡単に取得できますenumitem
。
\documentclass{book}
\usepackage{enumitem}
\setlength\parindent{1em}
\begin{document}
\begin{description}[style=nextline, listparindent=\parindent, parsep=0pt]
\item[The First Item]This text describes the first item.
\item[The Second Item]This text describes the second item.
Contrary to the description of the first item, it is quite a bit longer.
Additionally, it consists of multiple paragraphs, which causes problems
with redefining the item command.
\end{description}
\end{document}