Transmissivity is a measure of the ability of a porous medium, such as soil or rock, to transmit water. It is defined as the hydraulic conductivity of the medium multiplied by its thickness. In other words, it is the rate at which water can move through a unit width of the porous medium under a unit hydraulic gradient. Transmissivity is commonly used in hydrogeology to characterize the properties of aquifers and to model groundwater flow. It is typically expressed in units of length squared per time, such as meters squared per day or feet squared per second.