Saddle

Edit and compile if you like:

\documentclass[svgnames]{standalone}

\usepackage{pgfplots}
\pgfplotsset{compat=newest}

\begin{document}

\begin{tikzpicture}
  \begin{axis}[samples=30,ticks=none,xlabel={$V$},ylabel={$T$},zlabel={$F(T,V)$}]

    \addplot3[surf,color=DarkBlue,opacity=0.5,domain=-2:2,faceted color=black] {x^2-y^2};
  \end{axis}
\end{tikzpicture}

\end{document}