This is how I format my definitions in LaTeX:
\documentclass{article}
\usepackage{color,amsmath,amssymb}
\usepackage{framed}
\makeatletter
\newdimen\errorsize \errorsize=0.2pt
% Frame with a label at top
\newcommand\LabFrame[2]{%
\fboxrule=\FrameRule
\fboxsep=-\errorsize
\textcolor{FrameColor}{%
\fbox{%
\vbox{\nobreak
\advance\FrameSep\errorsize
\begingroup
\advance\baselineskip\FrameSep
\hrule height \baselineskip
\nobreak
\vskip-\baselineskip
\endgroup
\vskip 0.5\FrameSep
\hbox{\hskip\FrameSep \strut
\textcolor{TitleColor}{\textbf{#1}}}%
\nobreak \nointerlineskip
\vskip 1.3\FrameSep
\hbox{\hskip\FrameSep
{\normalcolor#2}%
\hskip\FrameSep}%
\vskip\FrameSep
}}%
}}
\definecolor{FrameColor}{rgb}{0.25,0.25,1.0}
\definecolor{TitleColor}{rgb}{1.0,1.0,1.0}
\newenvironment{contlabelframe}[2][\Frame@Lab\ (cont.)]{%
% Optional continuation label defaults to the first label plus
\def\Frame@Lab{#2}%
\def\FrameCommand{\LabFrame{#2}}%
\def\FirstFrameCommand{\LabFrame{#2}}%
\def\MidFrameCommand{\LabFrame{#1}}%
\def\LastFrameCommand{\LabFrame{#1}}%
\MakeFramed{\advance\hsize-\width \FrameRestore}
}{\endMakeFramed}
\newcounter{definition}
\newenvironment{definition}[1]{%
\par
\refstepcounter{definition}%
\begin{contlabelframe}{Definition \thedefinition:\quad #1}
\noindent\ignorespaces}
{\end{contlabelframe}}
\makeatother
\begin{document}
\begin{definition}{Quadratic Equation}
A Quadratic Equation is an equation in the form:
\begin{equation}
ax^2+bx+c=0
\end{equation}
where \(a,b,c\in\mathbb{R}\).
\end{definition}
\end{document}
and this is what you get:
