\newtheorem{def} throws an error

the following is a piece of the code I am working on. It says that there is an error with \newtheorem{def} but I really don't understand why. Thank you very much for your help!

\documentclass[12pt]{article}
\usepackage[margin=1in]{geometry}  % set the margins to 1in on all sides
\usepackage{graphicx}              % to include figures
\usepackage{amsmath}               % great math stuff
\usepackage{amsfonts}              % for blackboard bold, etc
\usepackage{amsthm}                % better theorem environments
\usepackage{esint}          


\newtheorem{thm}{Theorem}[section]
\newtheorem{lem}[thm]{Lemma}
\newtheorem{prop}[thm]{Proposition}
\newtheorem{cor}[thm]{Corollary}
\newtheorem{conj}[thm]{Conjecture}

\newtheorem{def}[thm]{Definition}

It throws an error even if I do not include "\begin{def}, \end{def}" in my code.