Nucleophilicity refers to the strength of a nucleophile, which is a species that donates an electron pair to form a chemical bond in a reaction. It indicates how readily a nucleophile can attack an electrophile and is influenced by factors like charge, electronegativity, and solvent effects. The concept plays a critical role in reactions involving heterocyclic aromatic compounds, the synthesis of amines, functional group interconversions, and the basicity and structure of amines.