How to edit form of Definition

Question

In my project, in the section 1.1, I want to create a definition. When I use

\begin{definition} abc \end{definition}

the output goes as "Definition 1.11. abc"

I want to change the format of my definition to "1.1.1. Definition. abc"

How can I do that ? Thank you.

Answer 1

If you're using one of the standard document classes, your current preamble code likely contains an instruction similar to

\newtheorem{theorem}{Theorem}[chapter]

I suggest you change it to

\newtheorem{theorem}{Theorem}[section]

If that doesn't work for you (possibly because you use an uncommon document class), I suggest you execute

\counterwithin{theorem}{section}

after setting up the theorem-like environments.

---

\documentclass{report} % or some other suitable document class
\usepackage{amsthm}
\newtheorem{theorem}{Theorem}[section]
\newtheorem{lemma}[theorem]{Lemma}
\theoremstyle{definition} % optional
\newtheorem{definition}[theorem]{Definition}
\newtheorem{example}[theorem]{Example}
%\counterwithin{theorem}{section} % alternate solution
\begin{document}
\stepcounter{chapter} % just for this example
\stepcounter{section}
\begin{theorem}    aaa aaa \end{theorem}
\begin{lemma}      bbb bbb \end{lemma}
\begin{definition} ccc ccc \end{definition}
\begin{example}    ddd ddd \end{example}
\end{document}