diffuser

UK: /dɪˈfjuːzə/ US: (di fyo̅o̅′zər)

WordReference Random House Unabridged Dictionary of American English © 2025

dif•fus•er  (di fyo̅o̅′zər), n. 

1. Physics a person or thing that diffuses.
2. Physics (in various machines or mechanical systems, as centrifugal pumps or compressors) a device for utilizing part of the kinetic energy of a fluid passing through a machine by gradually increasing the cross-sectional area of the channel or chamber through which it flows so as to decrease its speed and increase its pressure.
3. Physics, Electricity (in a lighting fixture) any of a variety of translucent materials for filtering glare from the light source.
4. Physics, Civil Engineering a pierced plate or similar device for distributing compressed air for aeration of sewage.

Also, diffusor. 

• diffuse + -er¹ 1670–80

Collins Concise English Dictionary © HarperCollins Publishers::

diffuser, diffusor / dɪˈfjuːzə/ noun

1. a person or thing that diffuses
2. a part of a lighting fixture consisting of a translucent or frosted covering or of a rough reflector: used to scatter the light and prevent glare
3. a cone, wedge, or baffle placed in front of the diaphragm of a loudspeaker to diffuse the sound waves
4. a duct, esp in a wind tunnel or jet engine, that widens gradually in the direction of flow to reduce the speed and increase the pressure of the air or fluid
5. a light-scattering medium, such as a screen of fine fabric, placed in the path of a source of light to reduce the sharpness of shadows and thus soften the lighting
6. a device, attached to a hairdryer, which diffuses the warm air as it comes out