Dictionary Definition:

“[An element is considered electronegative if it ] tends to acquire electrons and form negative ions in chemical reactions.”

Selected from OxfordDictionaries.com

Functional Definition:

Electronegativity is the amount of attraction a molecule has to the lone pair electrons of another molecule.

The more electronegative atom in a molecule usually takes on a partial negative charge because the electrons spend more time near its end of the bond. This gives the less electronegative atom a partial positive charge.