Enthalpy refers to the energy needed to ionize one mole of isolated molecules in the gaseous state. It is the ease at which a substance can be ionized (i.e., remove electrons from it in order to make positive ions).
Electron Gain Enthalpy refers to the amount of energy that is released when one mole or electron is added to gaseous elements of an element.
Electron Affinity can be described as Electron Gain Enthalpy, but it is viewed as energy that is supplied to the environment rather than the atoms. Its sign is therefore opposite to the former.
These two terms describe how easy it is to add an electron to an element’s atoms. Remember that electron affinity and ionisation energy are the energy provided to the surrounding environment.