[Unimportant things]

\theoremstyle{definition}
\newtheorem{definition}{Definition}
	\newcommand{\bdefbs}[1]{\begin{definition}[{#1}]\\} % <- This line cannot be broken.
	\newcommand{\edefbs}{\end{definition}\begin{flushright} $\mathop{\bigstar}$ \end{flushright}}

\begin{document}

\bdefbs
Here I write some definition.
\edefbs

\end{document}