What makes a nano a nano?

a lot of people classify a reef tank of 30 gallons or under a nano. (though there are people that go a bit more each way.)

basically its a smaller reef tank.
 
Is it a nano considering just the tank size or the volume of the sump/fuge, if you have one, as well?
 
is the size, for what i hear was a company that start calling their small tanks "nano" so it got like a general name for display tanks under 30g, and if im not wrong under 2.5g are picos
 
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