Valence Electron
Definition
The electrons in an atom's outermost shell: the ones that actually do the chemistry of bonding and reacting.
Technical definition
Valence electrons are the electrons in the outermost principal energy level of an atom. Their count determines an element's group, bonding capacity, and typical ion charge.
Example
Oxygen has 6 valence electrons, so it typically gains 2 to reach 8. That is why oxide is O^2- and water is H2O.
Common confusion
Valence electrons are only the outer-shell ones. Carbon's inner 1s^2 electrons never participate in bonding, so carbon has 4 valence electrons, not 6 total.