Layering is the build-up of distinct strata on a surface, where each layer sits over and partially conceals or modifies the ones beneath.
In painting, layering covers the whole sequence of underpainting, modelling, glazing, and scumbling that produces depth of colour and surface. In collage and digital image-making the term is more literal: separable elements stacked in a definite z-order, each with its own edge and opacity. Description of a layered picture asks how many layers are legible, how cleanly they read as stacked, and whether the lower layers are meant to show through (as in glazing) or to be buried (as in opaque overpainting). Layering produces a particular kind of pictorial time — the upper layers were placed last, the lower first — which often shows even when not intended.