Current datasheet section 30.2 table 30.1 for a chip run at 5v ,
gives
low as < than (VIL - first row) as 0.3Vcc (aka 0.3 * Vcc) or 1.5V and
high as >than (VIH - second row) as 0.6Vcc (aka 0.6 * Vcc) or 3.0V
With limits for how far bellow ground and above Vcc you can go.
Anything in between "floats" and gives an undefined result when read.
The xVcc notation instead of x * Vcc has been known to cause problems.
This is nowhere near dlloyd 's figures
Researching this forum, most replies mention 0.3Vcc for LOW (logic 0) and 0.6Vcc for HIGH (logic 1).
Mark
PS The datasheet link in the Uno's documentation no long works (Atmel have moved and rewritten the datasheet.)
M