A nucleophile is any negative ion or neutral molecule whose electronic configuration consists of at least one unpaired electron pair. In chemical reactions, the nucleophile attacks the positive center of some other molecule or positive ion, and then donates a pair of electrons to form a covalent bond. Nucleophiles are produced in the formation of acids and bases, and in covalent carbon bonding in organic compounds.

Thus, reagents that seek a proton or some other positive center are called nucleophiles (from nucleus, i.e., the positive part of the atom, plus phile from the Greek word for love). Electrophiles, on the other hand, are reagents that, in their reactions, seek extra electrons to achieve a stable valence shell of electrons. For example, in their reactions, nucleophiles consisting of ions with a negatively charged carbon atoms seek a proton or some other positive center to which they can donate their electron pair and thereby achieve electrical neutrality. In a similar way, both hydroxide ions and water molecules may act as nucleophiles by reacting with alkyl halides to produce alcohols.

In nucleophilic substitution reactions, the unshared pair of electrons on the nucleophile is used in a new carbon-nucleophile bond. In the case of nucleophilic substitution reactions involving the polar carbon-halogen bond of an alkyl halide and a negative nucleophile, the nucleophile seeks the carbon atom that bears the halogen atom. This carbon has a partial positive charge because the halogen atom pulls the electrons in the carbon-halogen bond toward itself. The halide retains the electron pair of the carbon-halogen bond when it leaves. Such leaving groups are usually less nucleophilic than the reacting nucleophile, so these reactions usually do not immediately reverse themselves.