I want to highlight my proposition and i don't need the enumeration for now.

The source file is the following:

\usepackage{amsmath, amssymb, amsthm}

\begin{document}

\begin{theorem}

... bla bla bla, not so interesting ...

\end{theorem}

\end{document}


To compile I use pdflatex, and the reported error is "Environment theorem is undefined".

You have to know that I'm sure that the other math packages are installed (amsmath, amssymb), and secondly my proposition isn't highlighted in the final document.

EDIT:

For highlighting, I wish that the reader can understand almost without reading that a block of text is a theorem, a proposition, and so on... So, what I want is for example, that all the propositions have the same style, all the theorems have the same style (but different from the style of the propositions). And I think that using this package is fast and handy.

• It's not enough to load the package; you need to declare the structure first, as in \newtheorem{theorem}{Theorem} or \newtheorem*{theorem}{Theorem} (for unnumbered theorems). – Gonzalo Medina Jul 22 '14 at 16:11
• What do you mean with "highlighted"? Do you want a special formatting such as a background color, a frame? Please describe exactly what you are trying to achieve. – Gonzalo Medina Jul 22 '14 at 16:13

Loading amsthm alone it's not enough. You need to declare the theorem structure first, using \newtheorem (or the starred variant for unnumbered structure), as in

\documentclass{article}
\usepackage{amsmath, amssymb, amsthm}

\newtheorem{theorem}{Theorem}

\begin{document}

\begin{theorem}
A test theorem
\end{theorem}

\end{document}


The result:

\newtheorem has a more complex syntax with two optional (mutually exclusive) arguments allowing control over the counter; refer to the package documentation for details.

Regarding the second part of your question, you mentioned that you want to "highlight" the theorems, but you didn't mention what exactly do you mean by highlighting. Do you want a special formatting such as a background color, a frame? If so, please refer to mdframed or thmtools or tcolorbox. The documentation of each package has example showing how to get fancy designs for theorems and in this site there are numerous examples of the use of those packages to produce theorem-like structures with several kinds of decorations: you could have a look, for example, at