Typesetting a definition

Question

My current set up for type setting definitions is

\newenvironment{definition}[1][Definition:]{\begin{trivlist}                         
  \item[\hskip \labelsep {\bfseries #1}]}{\end{trivlist}}

which produces

I don't really like this look but I don't want my definitions to look like the my physicals laws which are done in the mdframed package with the option framemethod = tikz

\begin{mdframed}[backgroundcolor = gray!30,
  frametitle = Oberth effect]
  Is where the use of a rocket engine when traveling at high speed generates
  much more useful energy than one at low speed.
\end{mdframed}

and look like

Are there any suggestions on what I can do have the definition environment not be so plain but not mimic the look of the mdframed environment?

\documentclass{article}
\usepackage[framemethod = tikz]{mdframed}
\newenvironment{definition}[1][Definition:]{\begin{trivlist}                         
      \item[\hskip \labelsep {\bfseries #1}]}{\end{trivlist}}
\begin{document}
\begin{definition}
This is a definition.
\end{definition}
\begin{mdframed}[backgroundcolor = gray!30,
  frametitle = Oberth effect]
  Is where the use of a rocket engine when traveling at high speed generates
  much more useful energy than one at low speed.
\end{mdframed}
\end{document}

Answer 1

Some options of an endless selection (I chose to restrict the color to gray shades so to keep some consistency with the existing environment and to keep the document sober); I used mdframed for some examples and tcolorbox for others; any of the examples could have been produced with either of these two powerful packages.

First, using mdframed:

\documentclass{article}
\usepackage{amsthm}
\usepackage[framemethod=tikz]{mdframed}
\usepackage{lipsum}

\newtheoremstyle{defi}
  {\topsep}%
  {\topsep}%
  {\normalfont}%
  {}%
  {\bfseries}% 
  {:}%
  {.5em}%
  {\thmname{#1}\thmnote{~(#3)}}%
\theoremstyle{defi}
\newmdtheoremenv{definitioni}{Definition}
\newmdtheoremenv[
hidealllines=true,
leftline=true,
innertopmargin=0pt,
innerbottommargin=0pt,
linewidth=4pt,
linecolor=gray!40,
innerrightmargin=0pt,
]{definitionii}{Definition}
\newmdtheoremenv[
roundcorner=5pt,
innertopmargin=0pt,
innerbottommargin=5pt,
linewidth=4pt,
linecolor=gray!40,
]{definitioniii}{Definition}

\begin{document}

\begin{mdframed}[backgroundcolor = gray!30,
  frametitle = Oberth effect]
  Is where the use of a rocket engine when traveling at high speed generates
  much more useful energy than one at low speed.
\end{mdframed}

\begin{definitionii}
\lipsum[4]
\end{definitionii}
\begin{definitioni}
\lipsum[4]
\end{definitioni}
\begin{definitioniii}
\lipsum[4]
\end{definitioniii}

\end{document}

Another option:

\documentclass{article}
\usepackage{amsthm}
\usepackage[framemethod=tikz]{mdframed}
\usepackage{lipsum}

\newtheoremstyle{defi}
  {\topsep}%
  {\topsep}%
  {\normalfont}%
  {}%
  {\bfseries}% 
  {:}%
  {.5em}%
  {\thmname{#1}\thmnote{~(#3)}}%
\theoremstyle{defi}
\newmdtheoremenv[
hidealllines=true,
innertopmargin=0pt,
linewidth=4pt,
linecolor=gray!40,
singleextra={
  \draw[line width=3pt,gray!50,line cap=rect] (O|-P) -- +(1cm,0pt);
  \draw[line width=3pt,gray!50,line cap=rect] (O|-P) -- +(0pt,-1cm);
  \draw[line width=3pt,gray!50,line cap=rect] (O-|P) -- +(-1cm,0pt);
  \draw[line width=3pt,gray!50,line cap=rect] (O-|P) -- +(0pt,1cm);
  },
firstextra={
  \draw[line width=3pt,gray!50,line cap=rect] (O|-P) -- +(1cm,0pt);
  \draw[line width=3pt,gray!50,line cap=rect] (O|-P) -- +(0pt,-1cm);
},
secondextra={
  \draw[line width=3pt,gray!50,line cap=rect] (O-|P) -- +(-1cm,0pt);
  \draw[line width=3pt,gray!50,line cap=rect] (O-|P) -- +(0pt,1cm);
}
]{definitioni}{Definition}

\begin{document}

\begin{mdframed}[backgroundcolor = gray!30,
  frametitle = Oberth effect]
  Is where the use of a rocket engine when traveling at high speed generates
  much more useful energy than one at low speed.
\end{mdframed}

\begin{definitioni}
\lipsum[4]
\end{definitioni}

\end{document}

Another option:

\documentclass{article}
\usepackage{amsthm}
\usepackage[framemethod=tikz]{mdframed}
\usepackage{lipsum}

\newmdenv[
innertopmargin=0pt,
roundcorner=5pt,
linewidth=1pt,
linecolor=gray!40,
innertopmargin=\baselineskip,
singleextra={
  \node[
    anchor=west,
    xshift=7pt,
    fill=gray!20,
    rounded corners=2pt,
    draw] at (P-|O) {\bfseries Definition:};
},
firstextra={
  \node[
    anchor=west,
    xshift=7pt,
    fill=gray!20,
    rounded corners=2pt,
    draw] at (P-|O) {\bfseries Definition:};
}
]{definition}

\begin{document}

\begin{mdframed}[backgroundcolor = gray!30,
  frametitle = Oberth effect]
  Is where the use of a rocket engine when traveling at high speed generates
  much more useful energy than one at low speed.
\end{mdframed}

\begin{definition}
\lipsum[4]
\end{definition}

\end{document}

And now some more options using the tcolorbox package:

\documentclass{article}
\usepackage{amsthm}
\usepackage[framemethod=tikz]{mdframed}
\usepackage[most]{tcolorbox}
\usepackage{lipsum}

\newtcolorbox{definitioni}{
  breakable,
  fonttitle=\bfseries,
  title={Definition:}
}
\newtcolorbox{definitionii}{
  skin=enhanced,
  breakable,
  colback=white,
  colframe=gray!30,
  coltitle=black,
  fonttitle=\bfseries,
  leftrule=0.5cm,
  arc=0pt,
  outer arc=0pt,
  title={Definition:}
}
\newtcolorbox{definitioniii}{
  skin=enhanced,
  breakable,
  fonttitle=\bfseries,
  colback=white,
  arc=0pt,
  outer arc=0pt,
  title={Definition:},
  shadow={1mm}{-1mm}{0mm}{fill=gray,opacity=0.5,sharp corners}
}

\begin{document}

\begin{mdframed}[backgroundcolor = gray!30,
  frametitle = Oberth effect]
  Is where the use of a rocket engine when traveling at high speed generates
  much more useful energy than one at low speed.
\end{mdframed}

\begin{definitioni}
\lipsum[4]
\end{definitioni}
\begin{definitionii}
\lipsum[4]
\end{definitionii}
\begin{definitioniii}
\lipsum[4]
\end{definitioniii}

\end{document}

And another one:

\documentclass{article}
\usepackage{amsthm}
\usepackage[framemethod=tikz]{mdframed}
\usepackage[most]{tcolorbox}
\usepackage{lipsum}

\newtcolorbox{definition}{
freelance,
breakable,
before=\par\vspace{2\bigskipamount}\noindent,
after=\par\bigskip,
frame code={
  \node[
  anchor=south west,
  inner xsep=8pt,
  xshift=8pt,
  rounded corners=5pt,
  font=\bfseries\color{white},
  fill=gray] at (frame.north west) (tit) {\strut Definition:};
  \draw[
  line width=3pt,
  rounded corners=5pt,gray
  ] (tit.west) -| (frame.south west) -- ([xshift=15pt]frame.south west);
},
interior code={},
top=2pt
}

\begin{document}

\begin{mdframed}[backgroundcolor = gray!30,
  frametitle = Oberth effect]
  Is where the use of a rocket engine when traveling at high speed generates
  much more useful energy than one at low speed.
\end{mdframed}

\begin{definition}
\lipsum[4]
\end{definition}

\end{document}