diamagnetic

Meaning: In a diamagnetic species every electron has a partner of opposite spin, so the spin magnetic moments cancel and there is no permanent net moment. An applied field induces a small opposing moment, so the sample is weakly repelled. A species with unpaired electrons is paramagnetic and is pulled into the field. That attraction is normally stronger than the weak repulsion that every species also shows.

On the MCAT: You are given an atom, ion, or diatomic molecule and asked whether it is diamagnetic or paramagnetic, or which of several species is. Write the electron configuration, fill degenerate orbitals singly first (Hund's rule), and count unpaired electrons. Zn, Zn²⁺, Ca, Ne, and Cu⁺ are diamagnetic. Fe³⁺ is paramagnetic. For transition metal cations, remove the 4s electrons before the 3d electrons. For diatomic molecules, use the molecular orbital diagram: N₂ is diamagnetic, while O₂ has two unpaired electrons in its π* orbitals and is paramagnetic. A passage may describe a sample pushed away from or pulled toward a magnet.

Common confusion: Diamagnetic does not mean unaffected by a magnet. It means weakly repelled. An even number of electrons also does not guarantee diamagnetism: O₂ has 16 electrons yet two are unpaired. Check how the electrons fill the orbitals, not just how many there are.