Diffusivity is a rate of diffusion, a measure of the rate at which particles or heat or fluids can spread.

It is measured differently for different mediums.

Diffusivity may refer to:

• Thermal diffusivity, diffusivity of heat
• Diffusivity of mass:
  • Mass diffusivity, molecular diffusivity (often called "diffusion coefficient")
  • Eddy diffusion, eddy diffusivity
• Kinematic viscosity, characterising momentum diffusivity
• Magnetic diffusivity

Diffusivity has dimensions of length² / time, or m²/s in SI units and cm²/s in CGS units.

• Diffusibility