\documentclass[]{amsart}

\usepackage{amsmath,amsthm,amscd,amsfonts,amssymb,enumerate,amsaddr}
\usepackage{graphicx}		
\usepackage{color}
\usepackage[colorlinks]{hyperref}

\newtheorem{theorem}{Theorem}[section]
\newtheorem{proposition}[theorem]{Proposition}
\newtheorem{lemma}[theorem]{Lemma}
\newtheorem{corollary}[theorem]{Corollary}
\theoremstyle{definition}
\newtheorem{definition}[theorem]{Definition}
\newtheorem{example}[theorem]{Example}
\theoremstyle{remark}
\newtheorem{remark}[theorem]{Remark}
\newtheorem{question}[theorem]{Question}
\numberwithin{equation}{section}

%%%***************************************************************************************************
\begin{document}

\title[]{the title of the article}

\author[M. Bagheri]{Maryam Bagheri}
\address[M. Bagheri]{address}
\email{sample@yahoo.com}

\keywords{$P$--space; zero set}
\subjclass[2010]{54G99; 54G10; 54C30}

\begin{abstract}

\end{abstract}
\maketitle

%%%%*************************************************************************************************
%%***************************************************************************************************
\section{Definitions and preliminaries}

\begin{definition}
\begin{itemize}
  \item [necessity]
   This is a math formula $x=Y$ for testing.
  \item [sufficiency]
  This is the second item.
\end{itemize}
\end{definition}

\end{document}