I've always thought of superfoods as those whole foods that are very high in bioavailable nutrients.
Another way to say it is, you can live on superfoods.
Here's what Wikipedia says:
Superfood is a term sometimes used to describe food with high phytonutrient content that some may believe confers health benefits as a result. For example, blueberries are often considered a superfood (or superfruit) because they contain significant amounts of antioxidants, anthocyanins, vitamin C, manganese, and dietary fiber.  However, the term is not in common currency amongst dieticians and nutritional scientists, many of whom dispute the claims made that consuming particular foodstuffs can have a health benefit There is no legal definition of the term and it has been alleged that this has led to it being over-used as a marketing tool.
That's what I call faint praise indeed. I suspect that the definition was shaped by Big Ag and the Pharmaceutical Industry.
The purpose of their definition is to invalidate or cast doubt. I don't know if that is really a fair approach.
Here is what Graci and Diamond, authors of The Power of Superfoods say:
Superfood the most nutrient-rich and completely absorbable food in any classification of protein, carbohydrates, fat or fiber;
contains powerful antioxidants, disease-preventing phytochemicals, and a wide range of colors;
allows the body a supply of balanced energy and supports accellerated healing;
examples are ...spirulina, phosphatidyl choline, alfalfa, barley, and wheat grasses, milk thistle, and ...green tea.
I don't know about phosphatidyl choline, but I've eaten all the other items.