How can I modify this,
\newenvironment{definition}[1][Definition]{\begin{mdframed}\begin{trivlist} \item[\hskip \labelsep {\bfseries #1}]}{\end{trivlist}\end{mdframed}} so that a newline (vertical space) is inserted after the word "Definition"? I want the content of my definition(s) to start below it.
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}
trivlisthere? What is your goal in the first place? There might be a much simpler solution.