Art direction
Also said as Visual direction, Art style, Visual identity
The set of decisions that makes every picture in a game look as if it belongs to the same game: palette, shapes, lighting, level of detail, how things move. An art director writes those rules and holds everyone to them.
Without it, ten good artists make ten good pieces that clash. A clear direction also costs less, because artists stop having to guess and a small team can look deliberate.
Say it
“Art direction for this game: three main colours and one accent, rounded shapes only, no outlines, light always from the top left. Anything that breaks those four rules goes back.”
Where you’ve seen it
Mirror's Edge is almost entirely white, with red marking the routes you can run. The art director's brief was reportedly to make the game recognisable at a glance, and one developer account says the clean city also answered motion sickness in the grittier early prototype.
Not to be confused with graphics
Graphics are what the hardware can draw. Art direction is what you choose to draw with it, and a strong direction often looks better on weaker hardware.