Overlays & Surfaces
Backdrop
also known as scrim, modal backdrop
Backdrop is the common name (used by Bootstrap and MUI) for the shaded layer rendered behind a modal or drawer. Clicking it usually dismisses the overlay. In Material Design's older 'backdrop' component the term also meant a background surface revealed behind a top app bar, but in everyday usage backdrop and scrim mean the same dimming veil.
Don't confuse it with
Backdrop and scrim are near-synonyms for the dimming layer. Bootstrap and MUI say 'backdrop'; Material Design says 'scrim'. Do not confuse either with the whole overlay, which includes the content surface on top.
In the wild
Bootstrap's modal renders a '.modal-backdrop' element behind the dialog.