Definition of Nucleophile
A nucleophile is a chemical species that is sufficient in electrons and has the ability to donate them. It has a low affinity for the electrons and is nucleus loving. It forms bonds by donating a pair of electrons to another chemical species that is called an electrophile which is known to be an electron acceptor. Due to their nature of donating electrons, they are called Lewis bases.
Most of the nucleophiles carry a negative charge or they may have a lone pair of electrons. The interaction between an electrophile and a nucleophile is brought by substitution and addition reactions. Examples of nucleophiles include anions such as Cl-, RS-, NH3 etc.