A ceiling air diffuser is a channeled cover placed over an air conditioning distribution duct in the ceiling of a room. The channels or slots in the diffuser direct air to flow in specific directions, thereby allowing cool air to go exactly where it is needed. Diffusers improve the efficiency of air conditioning and help to balance the temperature throughout a room. A reduction in the velocity of air movement yields more comfortable indoor spaces.

The most common ceiling air diffuser uses a pattern of concentric circles.

Modified designs can be used with drop ceilings. The structure can fit flush to the ceiling or protrude downward, as may be the case in large industrial spaces with high ceilings. Well-designed diffusers maximize air distribution while minimizing noise level.