The "external layer" refers to the outermost part of an object or system, providing protection and structure. In biological terms, it can describe the outer membrane of a cell, which safeguards the internal components while regulating what enters and exits. This layer is crucial for maintaining the integrity and functionality of the cell. In technology, the external layer can refer to the user interface of a software application, which is the part users interact with. It encompasses design elements like buttons and menus, ensuring a smooth user experience while hiding the complex processes happening behind the scenes.