Polarization perpendicular to the plane of incidence ($\mathbf{E}\perp \text{plane of incidence}$)

For reflection or refraction at a flat isotropic interface, s-polarization has its electric field perpendicular to the plane containing the incident wavevector and interface normal. At an isotropic dielectric boundary it is an eigenpolarization of the Fresnel reflection problem. The label is relative to the specified interface, not an absolute direction in space.