Layer mask
A greyscale mask attached to a single layer that controls its transparency pixel by pixel — white reveals, black hides — so you can conceal parts of a layer without erasing anything.
Also called: mask
A layer mask is painted directly onto one layer: white areas keep that layer visible, black areas hide it, and greys give partial transparency. Because nothing is deleted, you can repaint the mask at any time — the standard non-destructive way to blend a mockup element into a scene (e.g. fading a logo into a shadow).
It differs from a clipping mask: a layer mask uses its own painted greyscale, while a clipping mask borrows the shape of the layer beneath it.