Iron sulfide is a chemical compound (different from a mixture of Iron and Sulfur, show by its different physical and chemical properties). It is made of iron and sulfide ions.
A dielectric material (dielectric for short) is an electrical insulator that can be polarized by an applied electric field. dielectric resonator oscillator (DRO) is an electronic component that exhibits resonance of the polarization response for a narrow range of frequencies, generally in the microwave band. It consists of a puck of ceramic that has a large dielectric constant and a low dissipation factor.
A capacitor consists of two conducting surfaces, frequently referred to as plates, separated by an insulating layer usually referred to as a dielectric. The original capacitor was the Leyden jar developed in the 18th century. It is the accumulation of electric charge on the plates that results in capacitance.