Track matte

Two flavours. An alpha matte uses the transparency of the matte layer, so its shape becomes the window. A luma matte uses brightness, where white reveals and black conceals, which makes gradient wipes and soft edges straightforward. In practice you stack the matte layer above the layer you want revealed and point the lower one at it. Creators reach for this constantly: masking a presenter so a title appears to pass behind them, playing footage inside letterforms, and building soft-edged transitions out of a moving gradient. In After Effects it is the TrkMat column; other applications hide it under blending or compositing modes.