Posted 1 December 2016 - 09:33 AM
It can be a little arbitrary I guess. We consider it from the point of view of the main reaction centre. In carbonyl additions, the carbonyl carbon is an electrophilic and the main reaction centre. It's only possible for a nucleophile to attack this carbon, therefore it is nucleophilic attack.
In normal alkene addition, the alkene will act as the nucleophile, but it is the main reaction centre. Thus the thing adding to it is the electrophilic, hence electrophilic addition. While this is the normal case, it can be complicated by the presence of electron withdrawing groups attached to double bonds. Aromatic systems are also different.
In summary: it depends on what the main reactant is acting as. If it is the nucleophile, it's electrophilic addition, and if it's acting as an electrophile, it's nucleophilic addition.