A nucleophile is a species that can form a bond through the donation of a pair of electrons. The pair of electrons can be from a lone pair, a pi bond, or in certain cases, a sigma bond.
All nucleophiles are Lewis bases.
Many reactions of nucleophiles are irreversible. The relative strength of nucleophiles tends to be measured by reaction rate. This is in contrast to basicity, which is generally measured according to the position of an acid-base equilibrium.
Nucleophilicity is also more sensitive to steric effects than are acid-base reactions.