\documentclass[11pt]{article}
  \usepackage{fontspec}

\begin{document}

\section{Définitions}
\label{sec:definitions}

Lorem ipsum dolor sit amet, consectetur adipiscing elit.
Duis in auctor dui. Vestibulum ut, placerat ac, adipiscing
vitae, felis.

\section{Historique}

Tel que vu à la section \ref{sec:definitions}, on a...

\end{document}

%%% Local Variables:
%%% mode: latex
%%% TeX-engine: xetex
%%% TeX-master: t
%%% End: