Nucleophilic means "nucleus loving". A nucleophilic species has always two electrons in the form of a non-bonding doublet willing to donate to an electrophile, which always have an electron gap, thereby creating a new bond.

Bases are a subclass of nucleophiles, which donate their electrons specifically to protons.

The nucleophilicity of various species are compared by the reaction rates of reactions where these species act as nucleophiles.

There are four key factors contributing to nucleophilicity: Charge, size, electronegativity and solvent.