TL:DR – Superheated water is water that has been heated to above its boiling point while held under sufficiently high pressure to keep it liquid. Technically, it is at a temperature higher than its vaporization point at the absolute pressure where the temperature is measured. It is possibly to superheat liquids other than water for a range of industrial uses.
WARNING: DO NOT TRY THIS AT HOME
Put simply, superheating involves raising the temperature of a liquid, for instance, beyond its boiling point without allowing it to vaporize. This can be done by heating water in a sealed container above 100 Celsius. There is an urban myth that has done the rounds for many years that it is possible to superheat the contents of a liquid-filled cup in a microwave and trigger a geyser of fluid when you remove it and stir. Who hasn’t received the spam-mail describing the 26-year old who was severely disfigured by such an incident?
Well, there are risks associated with all cooking, and heating a liquid in a microwave for long enough will produce a boiling temperature liquid and a container surface coated with scalding hot “condensation” that could cause you to jolt and end up splashing yourself with scalding liquid.
Apparently, it happens, so be careful. However, I think it would be hard to actually “superheat” the liquid, although the guys in this video may have done just that using pure, distilled water or similar.
The liquid would have have no so-called “nucleation” points, specks of dust, particles, or whatever, even scratches inside the beaker to seed bubbles of steam. The water could very easily surpass its boiling point without actually boiling.
Like Snopes says, it is possible but takes a lot of effort to cause superheating in a normal cup under normal conditions in a microwave oven. Nevertheless, it’s not worth risking a scalding in an attempt to duplicate the above experiment with your morning coffee.