A special Adobe Photoshop layer that can be used to experiment with tonal adjustments such as levels and curves without permanently affecting underlying layers. Adjustment layers have two advantages over applying tonal adjustment commands alone: 1. Adjustment levels are dynamic. You can modify your settings or remove the layer altogether at any time you wish. 2. Tonal adjustment commands can only be applied to a single layer. On the other hand, an adjustment layer affects all layers underneath.