The only medium through which light can travel faster than in a vacuum or air is called a "dispersive medium" with a negative refractive index. In such a medium, the phase velocity of light can exceed the speed of light in vacuum (denoted by the symbol 'c').
One example of a medium with a negative refractive index is a material known as "left-handed metamaterial" or "negative index metamaterial." These metamaterials are artificially engineered to exhibit unique electromagnetic properties not found in natural materials. They are composed of precisely designed arrays of subwavelength structures that interact with electromagnetic waves in a way that results in negative refraction.
In left-handed metamaterials, the electric field, magnetic field, and wave vector of light are oriented in such a way that the material exhibits negative refractive index. This leads to an unusual phenomenon where the phase velocity of light is faster than 'c' in the medium, although the group velocity (which carries information) is still limited to 'c' and obeys the principle of causality.
It's important to note that left-handed metamaterials are a highly specialized and artificial construct, and they are not commonly found in nature. In most naturally occurring materials, including liquids and solids, the speed of light is slower than its speed in a vacuum.