Electronegativity of an element is a chemical property that describes the tendency of its atoms to attract electrons towards itself 1, for example in the context of the sharing of electrons involved in chemical bonding. The higher the electronegativity number of an element, the more it can attract electrons.
The most common way of quantifying the Electronegativity is that due to Pauling. This gives a dimensionless quantity, on a relative scale running from around 0.7 to 3.98 (hydrogen = 2.20). This is known as an electronegativity in Pauling units, which is what we use.
Values for the Pauling Electronegativity can be found tabulated for all elements in the Periodic Table.
The JSON schema and an example representation for this property can be found here.