Overlays & Surfaces
Scrim
also known as backdrop, overlay mask, shade
A scrim is a translucent sheet placed between the main content and an overlay to dim what's behind it, signaling that the background is temporarily inactive. It focuses attention on the foreground surface and often closes the overlay when tapped. Material Design calls it a scrim; many teams call it a backdrop.
Don't confuse it with
Scrim and backdrop usually mean the same dimming layer. The scrim is not the dialog itself, it's only the darkened area around it. A modal typically uses a scrim, but the scrim is just one part of the modal.
In the wild
The gray dimmed area around an open Instagram comment sheet is the scrim.