I don’t know about a rigorous scientific definition but how about processed compounds or naturally growing plant and animal foodstuffs which contain high levels of recognized beneficial vitamins, processed or natural supplements or (ethical or otherwise) drug structures? Things like some plants with high vitamins or minerals eg nuts, avocados, pistachios, and the lists go on from casual categories to arcane, select living species, eg insects.
The first part is basically most foods above the level of Twinkies. This part
"processed or natural supplements or (ethical or otherwise) drug structures? "
is just about anything, from vegetable powders (better to eat the actual veggie) to 'roids. What I've seen is that there's a new superfood every few years, then the hype/clickbait/marketing moves on. Somehow, the previous superfood becomes no longer super and turns into bad for you, or it is just ignored.
There are 'super foods' at your local grocery, the thing is they don't necessarily have the Cool Crowd aura. Example: Broccolli and collard greens are loaded with good for you stuff, just as much now as when your parents tried to get you to eat them. Throw some different colorful fruits that complement your meals onto your plate and you'll get the magic substance from whatever this, next and last year's miracle food is.
Nuts, seeds, etc are all part of a normal diet and have been since we lived in caves. I suppose one could posit an imaginary person who eats only cheeseburgers, fries, eggs and Pepsi and compare their diet to a healthy one, but that seems a rather pointless exercise.