Typically a graphical style where there are one two colors. (A bit can be either on or off, so two options) Commonly black and white, but can be any combination depending on the style the creator is trying to accomplish.
In the style of games made the 8-bit generation of game consoles & computers, or sometimes authentically created to run on original hardware. Common 8-Bit consoles include the Nintendo Entertainment System, Sega Master System, and the Commodore 64
Pixel Art refers to the charming, simplistic, retro/renaissance style of very-low-resolution graphics, such as from the 1977-1995 Atari, GameBoy, & Super Nintendo (SNES) eras.
