I call shoes “flats” when the heels are no higher than one inch. So flats actually come in an assortment of heel heights (as shown below). Even the flattest heels are around a quarter of an inch high.
It is hard to make an exact cutoff point, but normally when a heel reaches 1.5 inches I call the shoes low heels, and not flats. Flatforms, which are essentially platformed heels sans the arch, are also heels and not flats.
I’d like to hear your thoughts. Where do you draw the line?