Negative refraction refers to the electromagnetic phenomenon where a light ray is refracted at an interface in the reverse sense to that normally expected. V. G. Veselago proposed that a negative refraction can be obtained with a metamaterial, having a negative value for both (electric) permittivity ε and (magnetic) permeability μ. This material is also known as "double negative" material.
See the simulation at http://en.wikipedia.org/wiki/Negative_refraction
