For an insulator, the electrons are all tightly bound to the positive nuclei and are not free to move around. As a result, whenever we try to charge an insulator, there are patches of ions on the surface, but these patches are immobile.

Insulator: materials where electrons are not free to move around

Air is a really good insulator.

A Dielectrics is a type of insulator. But an insulator is not necessarily a dielectric.

Because, for example, a perfect insulator cannot be polarized