Those values are best defined and explained in the Atmel ATmega329p datasheet along with all the other electrical specifications for the chip.
Unfortunately, that didn't help me
Well, I didn't "say" that, but you asked about "0.5V"
In fact there's a point at which a voltage "must be" a HIGH and a similar point where some voltage "must be" a LOW, but there's a window of uncertainty between the two, a sort of no-man's land which should be avoided.
What voltage is that between? The uncertainty about "HIGH" and "LOW".
If you should say a voltage, were it would not anymore register a value as "HIGH" or the highest voltage you can think off, were it not at all would read it as "HIGH". (at least theoretically?)
Sorry... But I really need some sort of number, since I have made an error with an operational amplifier...
Hope there is some kind of possible answer