Thanks Jody. I knew it by velocity stack.I think the term diffuser can be a little confusing to those that think of something like a heat register or cooling duct grill or cover, it has a wide range of definition. In this case a diffuser is a device that controls airflow, they can be made and used to disperse air flow in a wide pattern or condense it into a tighter one. Either way the airflow is being controlled in a predictable manner.
A velocity stack or snorkel are other terms that may be more familiar.