grassbox
Clippings collection bag
A grassbox is the detachable container on a lawn mower that collects grass clippings as you mow.
Most walk-behind and riding mowers have a grassbox attachment, though some gardeners prefer to leave clippings on the lawn as mulch.
I need to empty the grassbox before I finish the back yard.
The term is more common in British English; Americans often say grass catcher or grass bag.