theorem

Synopsis:

\begin{theorem}
  theorem body
\end{theorem}

Produces ‘Theorem n’ in boldface followed by theorem body in italics. The numbering possibilities for n are described under \newtheorem (see \newtheorem).

\newtheorem{lem}{Lemma}      % in preamble
\newtheorem{thm}{Theorem}    
  ...
\begin{lem}                  % in  document body
  text of lemma
\end{lem}

The next result follows immediately.
\begin{thm}[Gauss]   %  put `Gauss' in parens after theorem head
  text of theorem
\end{thm}

Most new documents use the packages amsthm and amsmath from the American Mathematical Society. Among other things these packages include a large number of options for theorem environments, such as styling options.