1. A colloidal suspension of finely divided particles in a carrier liquid, usually an insulating oil, is which rheological properties are changed through an increase in resistance when an electric field is applied.
2. A class of liquid which stiffens into a semi-solid when subjected to a electric field.

Electrorheological fluids are commonly colloidal suspensions, and their stiffening under an electric field is reversible.

Under the electric field, electrorheological fluids form fibrous structures that are parallel to the applied field and can increase in viscosity by a factor of up to 105.

Lithium polymethacrylate is an example of an electrorheological fluid.