Diffuser (hair), an attachment that diffuses or spreads out air flow from a hair dryer.

Diffuser (cryptography), a method to protect against ciphertext manipulation attacks.

Diffuser (automotive), a shaped section of a car's underbody which improves the car's aerodynamic properties

Diffuser (band), a punk rock band from the Long Island area

Diffuser (sewage), an aerating device for sewage and industrial waste water treatment plants

Diffuser (breathing set part), a device fitted over an underwater breathing set's blowoff hole to break up the resulting bubbles

Diffuser (optics), a device that diffuses or spreads out or scatters light in some manner.

Diffuser (thermodynamics), a device that controls the characteristics of a fluid at the entrance to a thermodynamic open system. Commonly used in heating, ventilating, and air conditioning.