Yes - same thing [only a little different]
Dielectric Union is usually a full kit of parts.
Insulating Flange kit [usually] consists of insulating washers to put under one side's nuts, plastic sleeves for the studs, and an insulating gasket. You have to buy the pair of flange fittings.
They are not necessarily the same thing, but may serve very similar functions. duwe6 has explained insulating flange kits pretty well, and depending on application/materials/sizes etc. involved you will find various other sorts of devices that might well be referred to as "dialectric unions" with a good web search (I guess other key words that might help in this could be "insulated or "insulating" couplings).
