What is Nucleophile, electrophile, leaving group?

A nucleophile is a species that gives an electron pair to an atom other than hydrogen to form a bond (a Lewis base); an electrophile is a species that accepts such a pair (a Lewis acid, often a carbon bearing a partial positive charge). The nucleophilicity of a species is its reactivity as a nucleophile, measured by rate constants. A leaving group is the group that departs with the bonding pair when a bond to carbon breaks heterolytically.