In image-processing software applications (such as Adobe Photoshop, for example), a digital "overlay" used to keep image elements separate for ease of editing and manipulation. For example, a background may exist on one layer, while on top of it could be a figure intended to be superimposed on that background. By keeping the superimposed figure on a separate layer, it can be altered or deleted without disrupting the pixels forming the background.
In networking, layer refers to one of seven networking functions or aspects described in the OSI reference model. See Open Systems Interconnection.