A slightly slanted perspective (almost side view, isometric or similar) games where game is played on a mostly left or right progressing field with limited depth towards and away from the camera. Commonly used by beat 'em ups.
The first video game about Belt scroller was released in 1986.
The characters are commonly from directly side view perspective while the ground and environment itself is isometric.
Example from Golden Axe:
The name likely comes from the playfield resembling a conveyer belt.