Addition: Nucleophilic
Nucleophilic addition is a stepwise process. The first step involves a nucleophilic Lewis base (often with net negative charge) attacking a pi-system (usually at a position with partial positive charge) to generate an anion centre which subsequently complexes with an electrophilic Lewis acid.

pi-systems which are susceptible to nucleophilic addition usually possess one or more electronegative heteroatoms.

Note that it is often difficult to distinguish between nucleophilic and electrophilic addition. The first step of Grignard reagent addition to ketone functions involves the coordination of the carbonyl lone pair with the Mg ion. Thus the mechanism is actually electrophilic addition even though most texts regard it as nucleophilic.

For more information look in the Chemogenesis webbook section on addition reactions.