Gradient Mask
A mask that fades an adjustment smoothly along a linear direction, digitally imitating a graduated filter.
In depth
A gradient mask darkens a sky exactly as a glass grad would, but with the advantage that it can be repositioned and its transition adjusted after the fact. Its limitation is the same one the glass filter has: anything protruding above the horizon, such as a tree or a spire, is darkened along with the sky unless the mask is refined.
Examples in practice
A linear gradient darkening the top third of a landscape by one stop balances the sky against the foreground.