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.
This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. If an article link referred you here, you might want to go back and fix it to point directly to the intended page.