How do you number theorems but not definitions?

Question

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)?

Answer 1

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

Answer 2

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}