Electroretinography: A test in which the electrical potentials generated by the retina of the eye are measured when the retina is stimulated by light. Abbreviated ERG. In an ERG, an electrode is placed on the cornea at the front of the eye that measures the electrical response of the rods and cones, the visual cells in the retina at the back of the eye. An ERG may be useful in the evaluation of hereditary and acquired disorders of the retina. The instrument used to conduct ERG is an electroretinograph, and the resultant recording is called an electroretinogram.