what is creepage distance

Creepage is the linear surface distance from a live part to ground. HT insulators have stacked "skirt" or "bell" shapes to prevent dirt and dust from forming a conductive path across the insulator surface. Creepage is the total distance an electron from the live part would have to travel across the insulator surface (in and out of the bell area) to reach earth potential.

I don't think it needs to be to ground (earth), it could be to any other conductive path, i.e. phase-to-phase in a polyphase system.

You're correct. The path is to the nearest point which could produce a fault. Phase to phase would do that nicely.

Thanks for catching my error, JRaef!

We're almost there.

Creepage Distance – The shortest distance measured over the surface of insulation between conductive parts. Air gaps less than 1 mm (0.04 inch) are not considered to interrupt the surface (from UL913)

We often meet the problem, creepage. especially in HT.

It depends on two terminals voltage and insulation ( dielectric strength) and surface shape(curvature).

so when my workers wrap ht transformer, they have to take care to keep clear, clean to prevent from dust into its turns.