A dimensionless number, Pe, used in the study of the thermal transport phenomena of flowing fluids. For the diffusion of heat, it is expressed as the ratio of the molecular to convective heat transfer of a substance:

where v is the velocity of the substance, l is the distance travelled, and α is the thermal diffusivity. For the diffusion of mass, it is expressed as the ratio of the molecular and mass diffusion of a substance:

where D is the mass diffusion coefficient. It is named after French physicist Jean Claude Eugène Péclet (1793–1857).