surface layer

The surface layer is the topmost part of a material or environment where interactions occur most directly. In natural settings, like soil or water, it’s where the air, light, and temperature influence what happens beneath. In science or engineering, it refers to a thin outer region that affects how a material responds to forces, fluids, or other environmental factors. This layer often has different properties than the material inside, playing a key role in processes like protection, exchange of gases or nutrients, or surface reactions. It’s essentially the interface that mediates between the environment and the underlying material.