thickness direction; perpendicular direction; lengthwise direction
Technical term used in engineering, materials science, and manufacturing to refer to the direction along the thickness of an object, often perpendicular to the main surface. Can also refer to the lengthwise direction in some contexts, such as when describing the orientation of fibers or layers.
This material gains strength when compressed in the thickness direction.
By orienting the fibers in the lengthwise direction, durability is improved.