What is a small wall called?

A short or small wall that doesn't reach the ceiling is often called a pony wall, half wall, or knee wall.

A pony wall is a short wall. In different circumstances, it may refer to: a half wall that only extends partway from floor to ceiling, without supporting anything. a stem wall,

Retaining walls outdoors, hold back soil or water and are essential for preventing erosion on sloped terrains. Masonry walls are durable and visually appealing, made from bricks or stones bound with mortar. Non-load bearing walls simply divide spaces without carrying any structural load.