LAYER

A layer is a sheet or level within a layered system, typically defined by distinct properties, composition, or function relative to adjacent layers. Layers can be physical, such as a coating or stratum, or conceptual, such as levels in a hierarchical model.

In geology and earth science, layers or strata are formed by successive deposition of material, yielding sequences that can be dated and interpreted to reconstruct past environments. The study of these relationships, stratigraphy, relies on thickness, composition, and fossil content.

The atmosphere is divided into several layers, including the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Each layer exhibits characteristic temperatures, pressures, and chemical composition, influencing weather, radiation shielding, and communication signals.

Biology and medicine describe layered organization in tissues. The skin, for example, consists of multiple layers such as the epidermis and dermis, providing protection, barrier function, and sensory interfaces. Epithelial layers line organs and cavities, enabling selective transport.

In computing, layers describe interfaces and responsibilities within complex systems. Networking uses models such as the OSI seven-layer model and the TCP/IP suite, while software architectures often separate presentation, logic, and data layers. Neural networks are built from successive layers of neurons that transform inputs to outputs.

In graphics and materials science, layers are used to stack elements or coatings. Digital image editors employ layers to compose images non-destructively; multilayer coatings or thin films tailor optical, electrical, or protective properties.

The everyday sense of layering appears in clothing, food, or geological descriptions of successive layers within a soil profile. Across disciplines, layering supports organization, modularity, and functional specialization.