There's no special term, although you could just call it an outline, and there are several techniques that could be used.
A semi-automated method is to make a copy of the design, make sure everything is ungrouped until there are no groups left, and do Path > Union or use the shortcut Ctrl++
Then do Path > Dynamic Offset or use the shortcut Ctrl+J, and adjust the control to enlarge the shape
Then place it behind the artwork by sending it to the bottom of the stack.
Another method instead of adding a dynamic offset would be to apply a thick stroke to the United shape instead.
A more manual option is to draw around the shape with the Bézier tool, apply a fill, and again, place it behind the artwork. I like this method the best, as it's the simplest, and allows for total control of the outline.