What is electron gain enthalpy?

(N/A) Electron gain enthalpy $(\Delta_{eg}H)$ is defined as the change in enthalpy when an electron is added to a neutral gaseous atom $(X)_{(g)}$ to convert it into a negative ion $(X)_{(g)}^{-}$.
The process is represented as: $X_{(g)} + e^{-} \rightarrow X_{(g)}^{-}; \Delta_{eg}H$.
The value of $\Delta_{eg}H$ can be either negative or positive depending on the element.