\documentclass{report}

\usepackage{amssymb}
\usepackage{booktabs}

\begin{document}

\begin{table}
	\renewcommand{\arraystretch}{1.5}
	\centering
	\caption{Notations}
	\begin{tabular}{lp{7cm}l}
		\toprule
		Notation & Description & New\\
		\midrule
		$\mathrm{Pr}\{A\}$ & The probability that event A occurs & $\frac{\partial T}{\partial t} = \frac{\partial^{2} T}{\partial t^{2}}$\\
		$\mathbb{E}\{\Re\}$ & Expected value of the  & $\frac{\partial T}{\partial t} = \frac{\partial^{2} T}{\partial t^{2}}$ \\
		$\mathrm{H}$ & Entropy & $\frac{\partial T}{\partial t} = \frac{\partial^{2} T}{\partial t^{2}}$ \\
		\midrule
		$\lambda_{g}$ , $\lambda_{d}$ & Original and  arrival rate, respectively. & $\frac{\partial T}{\partial t} = \frac{\partial^{2} T}{\partial t^{2}}$\\
		$\mu_{g}, \mu_{d}$ & Original and dummy rate, respectively. & $\frac{\partial T}{\partial t} = \frac{\partial^{2} T}{\partial t^{2}}$\\
		\bottomrule 
	\end{tabular}
	\label{tab:symbols}
\end{table}

\end{document}