\documentclass{amsart}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{epsfig}
\usepackage{amsfonts}
\usepackage{amscd}
\usepackage{mathrsfs}
\usepackage{enumerate}
\usepackage{latexsym}

\newtheorem{theorem}{Theorem}[section]

\theoremstyle{definition}
\newtheorem{definition}[theorem]{Definition}

\begin{document}


\title[Title]{Title}

\author{author}


\begin{abstract}

Let ....

\end{abstract}

\maketitle

\section{Introduction}




\begin{definition}
Let ${\mathscr A}, {\mathscr B}, {\mathscr C}, {\mathscr D}, {\mathscr M}$  be a.....
\end{definition}


\end{document}