A diffuser is "a device for reducing the velocity and increasing the static pressure of a fluid passing through a system”.  The fluid's static pressure rise as it passes through a duct is commonly referred to as pressure recovery. In contrast, a nozzle is used to increase the discharge velocity and lower the pressure of a fluid passing through it.
Frictional effects during analysis can sometimes be important, but usually they are neglected. Ducts containing fluids flowing at low velocity can usually be analyzed using Bernoulli's principle. Analyzing ducts flowing at higher velocities with Mach numbers in excess of 0.3 usually require compressible flow relations.
A typical subsonic diffuser is a duct that increases in area in the direction of flow. As the area increases, fluid velocity decreases, and static pressure rises.