All dielectrics are insulators, but they’re insulators with special properties
Dielectric materials don’t pass current (insulator) but can be polarized so they pass the electric field. How well they can be polarized is represented in their dielectric constant. Materials that can be polarized better make for better material inside capacitors (the dielectric between plates) letting you get either a higher value or a smaller capacitor
To confuse things a bit, insulators (dielectric or otherwise) have a property called “dielectric strength” which isn’t related to the dielectric constant. Its the voltage/distance that will cause the material to break down and become a conductor. Air has a dielectric strength of 3MV/meter and is a terrible conductor normally but lightning exceeds the dielectric strength and creates a channel in the air.
Dielectrics are insulators. Any insulator has dialectic properties, but practical ones effectively transfer electric fields. Practical dielectrics can be made very thin in a strong electric field and still prevent current flow. In other words, they are excellent insulators, are not porous, and can be produced in thin films.
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