8 binary numbers (ie. 10110101, which is equal to the decimal 181) that form 1 bit. During the late 1970's up through the mid-late 1980's, most commercial CPU's could only count 8-bit numbers, ranging from 0-255.
Somehow, people only tend to think of video games when refering to 8-bit things, which it completely innacurate. Many computer systems, as well as video games, were concidered "8-bit."
8-bit processors were eventually replaced by more powerful and faster 16-bit CPU's such as IBM's 8080 line and Motorola's 68000.
x00 series, Commodore
series (PET 16, C-64, C-128, etc.), Apple
series, and other similar systems that used the 6502
, 6800, or Z-80 processors and their varients.
Game systems: Atari 2600 and 5200, Nintendo
Entertainment System (NES
), Nintendo's Gameboy
, Sega Genesis, Colecovision, etc.