Radiation stress is the flux of momentum carried by ocean waves.
The radiation stress is the momentum transferred through the water body per unit time (the flux of momentum) by wave orbital motion. It is called a stress because for obliquely incident waves, cross-shore momentum can be transferred by both cross-shore wave orbital motion and longshore wave orbital motion and longshore momentum can be transferred by both longshore wave orbital motion and cross-shore wave orbital motion. For obliquely incident waves, a cross-shore gradient in the wave orbital motion, for example due to wave breaking, will exert a stress on the water mass in cross-shore direction as well as in longshore direction. The stress in longshore direction generates a longshore current. The stress in cross-shore direction generates a water level set-up at the coast. Forcing by radiation stress gradients related to wave breaking is commonly an order of magnitude greater than forcing due to wind stress or other wave nonlinearities</ref>.
For a more detailed explanation, see Shallow-water wave theory#Radiation Stress (Momentum Flux) and Shallow-water wave theory#Radiation Stress Components for Oblique Waves.