Adjustment Layer
A layer containing a tonal or colour adjustment that affects the layers beneath it without altering their pixels.
In depth
Because the adjustment lives in its own layer, its settings, opacity, blend mode and mask can all be changed at any point. This turns editing into an arrangement of reversible decisions rather than a sequence of commitments, and it allows one correction to be applied to several layers or restricted to one.
Examples in practice
A curves adjustment layer at 60 per cent opacity applies a softened version of its contrast increase to everything below.