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: