Clipping Mask
A layer restricted so that it only affects the single layer directly beneath it, using that layer's shape as its boundary.
In depth
Clipping is a structural convenience: an adjustment clipped to a composited element travels with it, so moving or transforming the element carries its corrections along. It also prevents an adjustment intended for one part of a composite from leaking onto everything else in the stack.
Examples in practice
A saturation adjustment clipped to a replaced sky affects only that sky, even when other layers sit below it.