Revit calls the device that HVAC systems use to pass air through a ceiling or wall and connect to duct work and plenums - air terminals. They are much more commonly referred to as diffusers. Turns out that an air terminal is the sharp end of the stick in lightning protection systems. One engineer I met recently was a bit annoyed by the inappropriate use of the term.
Language is hard!!
According to Wikipedia:
Individual lightning rods are sometimes called finials, air terminals or strike termination devices.
Also according to Wikipedia:
Air terminals are the supply air outlets and return or exhaust air inlets. For supply, diffusers are most common, but grilles, and for very small HVAC systems such as in residences, registers are also used widely. Return or exhaust grilles are used primarily for appearance reasons, but some also incorporate an air filter and are known as filter returns.
So there you have it, both are correct! Rest easy knowing that if you call your diffusers air terminals or your air terminals diffusers you are good to go! Unless you do lightning protection too? Confused?