What is ionization enthalpy? What is the unit of ionization enthalpy?

(N/A) The quantitative measure of the tendency of an element to lose an electron is given by its ionization enthalpy.
"The energy required to remove an electron from an isolated gaseous neutral atom $(X)$ in its ground state is called ionization enthalpy."
$X_{(g)} + \Delta_{i}H = X_{(g)}^{+} + e^{-}$
Where $\Delta_{i}H$ is the first ionization enthalpy.
The $SI$ unit of ionization enthalpy is $kJ \ mol^{-1}$ or $J \ mol^{-1}$.