The terms air diffuser, register, grille, and vent are often used interchangeably when it comes to heating, ventilation, and air conditioning (HVAC) systems. However, there are some subtle differences between these terms.

An air diffuser is a device that is installed at the end of an HVAC duct to distribute air into a room. It might have an adjustable damper to control the direction of airflow.

A register is a grille that includes a damper to control the amount of air flowing through it. Registers are typically installed on the floor, wall, or ceiling and are designed to cover the duct opening.

A grille is a decorative cover for an air duct that allows air to flow through while also preventing larger objects from entering the duct. Grilles are typically stationary and do not have adjustable dampers.

A vent is a term used to describe any opening through which air can flow, such as a window or a door. In the context of HVAC systems, a vent is usually used to refer to the opening where the air is released into a room.

In summary, air diffusers and registers are similar in function, but registers have a damper to control airflow. Grilles are decorative covers that allow air to flow through, while vents are openings through which air can flow.