A Revit feature that bundles a set of elements together so they can be copied, placed, and updated as one unit across a project. Unlike a Family, which is a single reusable component, a Group bundles multiple existing elements (say, a hotel room's walls, door, and furniture) into one placeable unit - editing one instance of the group can propagate to every copy, which makes it a fast way to repeat identical layouts across a floor or a whole tower.