dielectric
A dielectric is an insulating material placed inside a capacitor to make it store more charge. Glass, plastic, ceramic, mica, and even paper or air act as dielectrics. Slip such a slab between the plates of a capacitor and, for the same voltage, it can hold more charge, so the dielectric is the secret to packing large capacitance into a tiny component.
A dielectric works because its molecules polarize in an electric field: their positive and negative parts shift slightly apart, or existing molecular dipoles line up, so the material develops charge layers that partly cancel the field between the plates. This weakens the internal field by a factor called the dielectric constant (relative permittivity) kappa, greater than 1. With a dielectric, the capacitance rises to C = kappa times its vacuum value: a material with kappa = 5 multiplies the capacitance five-fold.
A dielectric does double duty. Besides raising capacitance, it physically separates the plates so they can sit very close without touching, and it withstands a strong field before breaking down, letting the capacitor tolerate higher voltage. Every material has a maximum field it can take, its dielectric strength; exceed it and the dielectric conducts and the capacitor fails, the same breakdown that makes a spark jump through air.
Sliding a plastic sheet with dielectric constant kappa = 3 between a capacitor's plates triples its capacitance, from say 90 pF to about 270 pF, at the same plate spacing.
A dielectric partly cancels the internal field, letting the capacitor store more charge per volt.
A dielectric is an insulator, so no free charge crosses it; its molecules only polarize. Push the field past the material's dielectric strength, though, and it breaks down and conducts, destroying the capacitor.