\documentclass{standalone}
\usepackage{mathtools}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}

\filldraw [fill=cyan!20] (0,0)..controls (2.5,-3)..(4,0);
\draw[dashed] (0,0)--(4,0);
\node at (2,-1) {$\Omega$};
\node at (2.5,-2.4) {$\Gamma$};
\fill (1,0) circle (.03) node[above]{$P$};












\end{tikzpicture}
\end{document}