\theoremstyle{definition}

\newtheoremstyle{break}
{\topsep}{\topsep}%
{}{}%
{\bfseries}{}%
{\newline}{}%
\theoremstyle{break}
\newtheorem{definizione}{Definizione}
\newtheorem{thm}{Teorema}
\newtheorem{cor}{Corollary}[thm]
\newtheorem{lem}[thm]{Lemma}