\documentclass[12pt]{article}
\usepackage [pagebackref=true, colorlinks, linkcolor=blue, citecolor=magenta, urlcolor=cyan] {hyperref}
\usepackage{xepersian}
\settextfont[Scale=1.05]{XB Niloofar}
\newtheorem{thm}{قضیه}[section]
\newtheorem{dfn}[thm]{تعریف}
\newtheorem{lem}[thm]{لم}
\newtheorem{prop}[thm]{گزاره}
\newtheorem{cor}[thm]{نتیجه}
\newtheorem{con}[thm]{حدس}
\newtheorem{notation}[thm]{نماد}
\newtheorem{rem}[thm]{تذکر}
\newtheorem{exm}[thm]{مثال}
\begin{document}
\begin{dfn}[صامت]
salam
\end{dfn}
\end{document}