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.
Dielectric grease is a special type of grease which is most commonly used for lubricating purposes. Dielectric grease is neutral in nature. It resists corrosion to a certain extent. It acts by restricting the entry of water and other agents into the electrical connector.