Zip cord is like loudspeaker cable but, particularly in the USA, was available in a heavier version that was commonly used to carry 110V AC to light appliences.
As far as I know is still in common use in the USA for lamps etc.

In Europe double-insulated cord has been standard for some time, however some old appliences may still have zip cord (or even twisted-pair), particularly old lamps and radios.

An American Zip cord extension lead:
[Linked Image from jlab.org]