A layer is one level or thickness of something stacked on top of another. When you make a sandwich, each ingredient forms a layer: bread, then peanut butter, then jelly, then more bread. When you dress warmly in winter, you might wear multiple layers of clothing, one over the other, to trap heat and stay comfortable.

Layers appear everywhere in nature and science. The Earth has layers: a thin crust on the outside, a thick mantle beneath it, and a core at the center. Ancient rock formations show layers of sediment built up over millions of years, like pages in a history book. The atmosphere has layers too, from the air we breathe near the ground to the thin upper reaches of space.

Artists use layers when they paint, building up colors one on top of another. Cakes often have layers separated by frosting. Computer programs work in layers of code. Even sounds can layer: in an orchestra, the melody from violins layers over the harmony from cellos and the rhythm from drums.

When you layer something, you're arranging it in levels, building it up gradually, one level at a time, to create depth and richness that wouldn't exist with just a single flat surface.