The invention relates to a diffusor having a wall (8) that encloses an inlet having a round cross-section that transitions into an angular cross-section on the outlet of the diffusor over the height of the wall (8) of the diffusor. The transitions (15) between the sides (34 to 37) of the wall (8) have a twist in the height direction that follows the twirl of the flow of air through the diffusor. The ventilator has such a diffusor. The device has a housing on which at least two ventilators, each having one diffusor, are arranged.