i) A nucleophile is a chemical species that donates an electron pair to an electrophile to form a chemical bond in realtion to a reaction.

ii) a nucleophilic reagent defined as the molecules or ions with a free pair of electrons or at least one pi bind can act as nucleophiles. For example OH^- Cl^-, CN^- etc

iii) Nucleophilic substitution reaction:- A nucleophile react with haloalkane having a partial positive on the carbon atom bonded to halogen. a substitution reaction take place and atom attached to the substrate departs as leaving atom or ion. Since the substitution reaction is initiated by a nucleophile it is called substitution reaction.For example nucleophillic substitution reaction of alkyl halides.