Define electronegativity? How does it differ from electron gain enthalpy?

Hint: The electronegativity and the electron gain enthalpy differ in the state of atom (free as in element or bound as in compound).

Complete step by step answer:
Electronegativity is defined as the tendency of the atom of an element in a chemical compound to attract a shared pair of electrons towards it in a covalent bond.
A number of electronegativity scales are present. These include Pauling scale, Milliken-Jaffe scale, Allred Kochow scale etc. Out of these scales, Pauling scale is usually used in which fluorine (the most electronegative element) was arbitrarily assigned a value of 4.0.

Electron gain enthalpy is defined as the enthalpy change when a neutral gaseous atom takes up an extra electron to form an anion.

Electron gain enthalpy is the tendency of an isolated gaseous atom to accept an additional electron to form a negative ion. Electronegativity is the tendency of the atom of an element in a chemical compound to attract a shared pair of electrons towards it in a covalent bond.
Electronegativity is a qualitative measure. It is not a measurable quantity.
However, electron gain enthalpy is a quantitative measure. It is a measurable quantity.

Note:
Electron gain enthalpy is closely related to electron affinity but significantly differs from electronegativity.Electron affinity is defined as the change in energy (in kJ/mole) of a neutral atom (in the gaseous phase) when an electron is added to the atom to form a negative ion.