Layer

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In Adobe Photoshop and similar image-editing applications, a layer is a transparent zone which stacks on top of zones below, and rests underneath zones above. When image pixels are place do a layer, they knock-out the pixels on the layers below and are themselves knocked out by any pixels directly above them on other, higher, levels.

Layers can be further specified for different types of blending and for different levels of opacity. Layers are often used in conjunction with masks.