A measure of the geometric complexity of a porous medium. Tortuosity is a ratio that characterizes the convoluted pathways of fluid diffusion and electrical conduction through porous media. In the fluid mechanics of porous media, tortuosity is the ratio of the length of a streamline—a flow line or path—between two points to the straight-line distance between those points. Tortuosity is thus related to the ratio of a fluid's diffusion coefficient when it is not confined by a porous medium to its effective diffusion coefficient when confined in a porous medium. Tortuosity is also related to the formation factor, which is the ratio of electrical resistivity of a conductive fluid in a porous medium to the electrical resistivity of the fluid itself.
In some literature, tortuosity denotes the square of the ratio defined above, whereas in other literature, the term tortuosity factor is used for the square of the ratio.