# How do you number theorems but not definitions?

I am using LaTeX theorem style where I have at the top:

\newtheorem{theorem}{Theorem}
\newtheorem{definition}{Definition}
\newtheorem{lemma}{Lemma}


and then I use

\begin{(theorem|definition|lemma)}
...
\end{(theorem|definition|lemma)}


This numbers the theorems, definitions, and lemmas separately. Is where a way to do it where the definitions aren't numbered (but still use the rest of the theorem style)?

-

You can get unnumbered theorem-like environment loading either amsthm or ntheorem and saying

\newtheorem*{definition}{Definition}


These two packages introduce also the concept of "theorem style" that helps to get, for example, upright text inside the definition environment. For amsthm it would be

\theoremstyle{definition}
\newtheorem*{definition}{Definition}


and similarly for ntheorem

-

With the front-end package thmtools (and amsthm or ntheorem as back-ends), the corresponding definitions for the two structures would be

\documentclass{article}
\usepackage{amsthm}
\usepackage{thmtools}
\declaretheorem{theorem}
\declaretheorem[numbered=no]{definition}

\begin{document}

\begin{theorem}
Test
\end{theorem}

\begin{definition}
Test
\end{definition}

\end{document}


The definition pre-defined style from amsthm can be used by saying

\declaretheorem[style=definition,numbered=no]{definition}

-