It's the size of the tank. That in turn affects the choice of plants and the method of filtration, or the decision not to filter.
In a nonmathematical context such as this, nano means extremely small. Smaller than small. XS. If nano got bigger, it would be small.
Mathematically, it means one billionth. 10x(-9).
So what all this really means is that nano is subjective. It's an opinion.
Some people may think nano tanks are 5 gal or less. Some people consider a 10 gal a nano tank. It really depends. What's the cutoff for a gigantic tank? What about a large tank? A medium tank? A small tank? Well, whatever's left is your opinion of what a nano tank is.
I think it's kinda generally accepted that a freshwater aquarium 5 gals and under is considered a nano. That's the common ground. Whether or not a 10 is included depends on where you think small begins.
"Insanity: doing [or asking] the same thing over and over again and expecting different results."