1) A measure of the relative tendency of an atom to attract electrons to itself when chemically combined with another atom.
2) A measure of the ability of an atom in a molecule to draw bonding electrons to itself. This is partially determined by how many electron vacancies are available in an element's filling orbital. The most electronegative elements are the halogens, which have only one vacancy (i.e. have seven electrons in their filling orbital). Sulfur and oxygen are also highly electronegative.
3) A number describing the attraction of an element for electrons in a chemical bond.