Dielectric is another name for insulator. Dielectric refers to any insulator’s ability to be polarized in a electric field. This means that bound charges in the material are rearranged to oppose the electric field. This polarization is called the permitivity of material. It has the units of Farads/meter.
A coax cable uses the permitivity of the material between the center conductor and sheild to determine its characteristic impedance when it is used as a transmission line.
The insulator in a capacitor is called a dielectric for the same reason. This polarization is why the addition of a dielectric to a cap increases its capacitance.