A layer is the following:

• In abstraction, a layer is an abstract place conceived as having depth.
• In telecommunications a layer is an F region.
• In telecommunications networks and open systems architecture a layer is a group of related functions that are performed in a given level in a hierarchy of groups of related functions.
• In object-oriented design a layer is a group of classes that have the same set of (link-time) module dependencies to other modules, i.e. a collection of reusable components that are available for reuse in similar circumstances.
• 2D_computer_graphics#Layers
• in archaeology, a layer is a body of soil-material, often containing artefacts, deposited at the same time. See stratigraphy.
• Dual layer recording for DVD.