because it's the corresponding height in ISA conditions?
No, it is the equivalent [bold]altitude[/bold] in ISA conditions. "Height" refers to distance above ground level, whereas "altitude" refers to distance above mean sea level (and for what it is worth, "elevation" refers to the altitude of the ground above sea level, i.e. Elevation + Height = Altitude)
So yes, Density Height is a misnomer technically, but somehow I don't see it going out of usage any time soon...
TT