Definitions are usually made with a theorem package like amsthm
. On the other hand, if you are not sure if you want some more fancy stuff later on, i would start with something based on tcolorbox
. The example shows the default box, it can be customized in various ways, to look more simple or to look even much more fancy.

\documentclass{article}
\usepackage{blindtext}
\usepackage{mdframed}
\usepackage{tcolorbox}
\newtcolorbox{definition}{title=Definition,}
%\usepackage{showframe}
\usepackage{amsthm}
\newtheoremstyle{phatty}{2ex}{2ex}{}{}{\bfseries}{}{\newline}{}
\theoremstyle{phatty}
\newtheorem*{pht}{Phatty}
\surroundwithmdframed{pht}
\begin{document}
\begin{definition}
\blindtext
\end{definition}
\begin{pht}
\blindtext
\end{pht}
\end{document}
trivlist
here? What is your goal in the first place? There might be a much simpler solution. – Johannes_B May 11 '16 at 6:03