nucleophilically

Meaning: A species acts nucleophilically when it uses a lone pair or pi bond to bond to an electrophile, usually an electron-poor carbon such as a carbonyl carbon or an alkyl carbon bearing a leaving group. Nucleophiles are anions (OH⁻, RO⁻, CN⁻) or neutral molecules with lone pairs (H₂O, NH₃, ROH). Nucleophilicity is a rate property. It rises with negative charge, falls with increasing electronegativity across a row (NH₂⁻ > OH⁻ > F⁻), and falls with steric bulk. In polar protic solvents it rises down a group (I⁻ > Br⁻ > Cl⁻ > F⁻), because small anions are tightly solvated by hydrogen bonding.

On the MCAT: A passage says an amine, alkoxide or hydroxide "attacks nucleophilically" at a carbonyl (addition, acyl substitution) or at the back side of an alkyl halide (SN2). You identify the nucleophile, place the curved arrow from its lone pair, rank nucleophiles, or predict how solvent or steric hindrance changes the rate.

Common confusion: Nucleophilicity is not basicity. Basicity is thermodynamic (affinity for H⁺, tied to pKa). Nucleophilicity is kinetic (how fast the species attacks an atom other than H, usually carbon). They often track together, but tert-butoxide is a strong base and a poor nucleophile because it is bulky, while I⁻ is a good nucleophile and a weak base. The atom being attacked is the electrophile, not the nucleophile.