The Brick Institute of America defines a cavity wall as having an air space between wythes greater than 2 inches. What do you call a wall system that has an open space between wythes less than 2 inches?
Most references define a cavity wall as a wall with a continuous air space between the wythes that is a minimum of 2 inches. In a wall system with a smaller air space, it is difficult to prevent mortar from bridging across the air space to the interior wythe. Walls with smaller open air spaces can be called drainage walls, if the air space can be kept open. By sloping the bed joints back away from the cavity when building the wall, avoiding mortar bridges is possible. It also is possible to use a drainage mat in smaller cavities to maintain an open space. Such drainage mats typically are used along foundation walls to create a space for water to flow to the drain tile. Drainage mats can allow some water to bridge across the air space, and some act as a vapor retarder, which may not be desirable. Unless special provisions are made to ensure that the cavity is kept clean, I recommend not using cavities that are less than 2 inches.