What defines a fast charger?

I don't think it is the current or the voltage that makes it a fast charger, true that you need more power than a typical charger to be a fast charger but let me explain what I think. Any charger that can charge a specific device fast should be called a fast charger. Now if I have a device with a 500mAH battery and have a charger that is 700mA and can charge the battery almost up to 80% in 40 min I think I can call that a fast charger!

On the other hand my nexus 7 2013 is charged via a 5.2V 1.35A charger and takes a pretty hefty 3.5-4hours so can I call it a fast charger? I don't think so. But this same 5.2V 1.35A charger can charge some other devices pretty fast.

So it is kind of a device specific thing, 5V 2.1A can be a fast charger or some newer ones that has different profiles like whenever you connect them with suitable devices they will provide more than 5V, about 9V-12V to deliver more power.

I think you have noticed that fast chargers will always say that it will reach 60-80% within this time, haven't seen anyone saying it will reach 100% within this time so they can only manipulate the constant current part of the charging cycle and if any charger can do that I think it can be called a fast charger.