Fish paper or fishpaper is a strong, flexible, fibrous dielectric

Dielectric

A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...

 paper. It resists moderate heat and mechanical injury, and is often used for wrapping coils and insulating stove-top parts. It is hygroscopic and so must be treated with paraffin for use in moist environments. Some fish papers incorporate mica

Mica

The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...

 layers to increase the dielectric strength

Dielectric strength

In physics, the term dielectric strength has the following meanings:*Of an insulating material, the maximum electric field strength that it can withstand intrinsically without breaking down, i.e., without experiencing failure of its insulating properties....

while still giving good mechanical strength.