A nucleophile is a chemical species that donates an electron pair to form a new chemical bond. It is characterized by its electron-rich nature and its ability to participate in nucleophilic reactions. In Lewis acid-base theory, a nucleophile is considered a Lewis base, as it can donate electrons to form a coordinate covalent bond.

Lewis acid: A Lewis acid is an electron pair acceptor. It has an empty orbital that can accept a pair of electrons. This is the opposite of what a nucleophile does.

Bronsted acid: A Brønsted acid is a proton (H^+) donor. While some nucleophiles can also act as Brønsted bases (proton acceptors), the defining characteristic of a nucleophile is its ability to donate an electron pair, not necessarily accept a proton.

Bronsted base: A Brønsted base is a proton (H^+) acceptor. While many nucleophiles are also Brønsted bases because they possess lone pairs of electrons that can accept a proton, the fundamental definition of a nucleophile revolves around electron pair donation.

Nucleophile: The term "nucleophile" means "nucleus-loving". Nucleophiles are electron-rich species that are attracted to positive charges or electron-deficient centers (nuclei). They donate a pair of electrons to form a new covalent bond.