I first heard the word "nano" being applied to aquariums not long after biologists called most reef aquariums "micro reefs". They felt that every captive reef aquarium, regardless of size (even up to a thousand gallons) was still a micro-ecosystem and implied their difficulty of caretaking. Well many reefkeepers had already kept relatively successful 20 gallon, 10 gallon and even 5 and 2 1/2 gallon reef tanks so they figured theirs must be called "nano" and not "micro".
These days, I consider a nano tank to be an ecosystem that is small enough to be relatively unstable and requiring close attention and specific care. For a reef, this is anything under 20 gallons, but for a planted tank, 10 gallon tanks are actually quite stable planted ecosystems. Only when I've gotten down to 5 and especially 2 1/2 gallons have I run into considerable trouble stablizing the system.
I've got two "nano" planted tanks on my desk at work. One is the popular "Betta Vase" with one of those Lily Plants. The other is my attempt at a CO2 injected, high light (5.2 watts per gallon!) planted 2 1/2 gallon aquarium. In case you were wondering, the planted tank isn't growing much more than green slime algae.