From AMS Glossary
The opposition to transport of quantities such as water vapor and carbon dioxide to or from the stomata (pores) on the leaves of plants.
For water vapor, stomatal resistance r is defined as
where qi and qs are the specific humidities in the interior of the stomate cavity and at the exterior surface of the leaf, ρ is air density, and E is the moisture flux. The dimension of r is time per distance, that is, inverse velocity. This equation is analogous to that for an electrical resistance, proportional to the ratio of voltage potential divided by current flow.
Oke, T. R. 1987. Boundary Layer Climates. chap. 4.