Diffuser
From Wiki Weed
Part of many “heady glass” rig and water pipe, diffusers come in many configurations, but their purpose is the same: to add additional sites for the formation of bubbles as well as help with airflow. Diffusers and all the agitation that they create help to cool the smoke or vapor prior to inhalation, which makes it smoother on the lungs and often more flavorful. They generally are found dipping into the water at the bottom of the downstem, which is where the bowl connects to main part of the pipe.
