perpendicular

Equipotential surfaces are surfaces on which the potential is everywhere the same. No work is done in moving a charge over an equipotential surface. The electric field at an equipotential surface must be perpendicular to the surface since otherwise there would be a component of the field and also therefore an electric force parallel to the equipotential surface. Then work would be done in moving charges over the surface and the surface would therefore not be an equipotential surface.