A class of liquid which stiffens into a semi-solid when subjected to a electric field. Electrorheological fluids are most commonly colloidal suspensions, and their stiffening under an electric field is reversible. Under the electric field, electrorheological fluids form fibrous structures which are parallel to the applied field and can increase in viscosity by a factor of up to 10⁵. The stiffening of an electrorheological fluid is sometimes called the winslow effect after its first investigator, Willis Winslow in 1949. Electrorheological fluids can be characterized by their Mason number.

The effect has been proposed as a method of constructing shock absorbers on magnetically levitated trains. Lithium polymethacrylate is an example of an electrorheological fluid.