Key visual
A key visual (sometimes also referred to as key art or abbreviated as KV) is piece of art meant to depict the general tone/mood of the project. Key visuals are meant to attract attention and curiosity from a viewer to check out your project, so they are recommended to have even more thought put into them than even event CGs. Usually, the art is unique (not recycled game assets) and the different elements of the key art may be saved separately so that they can be used for different purposes. This art is usually used for promotional purposes (especially posters or box art) and/or in-game as the title screen/start screen. Though many additional pieces of promotional art may be produced, a key visual is distinctly intended to be the image that a person calls to mind when they think about the game.
