Overlays & Surfaces · Containers
Well
also known as inset container, sunken container
A well is a simple container (popularized by Bootstrap 3) that gives its content an inset, recessed appearance with a subtle background and border, visually separating it from surrounding content. It carries no strong meaning or urgency; it's just a neutral grouping surface. Bootstrap dropped it in v4 in favor of cards and utility classes.
See it live — shadcn/ui · MUI · Ant Design
Don't confuse it with
A well looks sunken/inset and is visually neutral; a card looks raised/elevated and represents one discrete item. A callout carries emphasis and meaning (tip, warning) that a well does not. A panel is a larger structural region.
In the wild
A grey recessed box wrapping a quote or code sample in a Bootstrap 3 site is a well.