The ratio of the speed of light in a vacuum to the speed of light in the material. By definition, the value must be greater than 1; in practice, it seldom goes above 4. For a given material, the refractive index varies with the wavelength of the light being transmitted (this is the mechanism behind dispersion of light through a prism) and temperature.
When a single value is reported, it is typically taken at the conventional default wavelength of 632.8nm at room temperature. This does not apply to materials that do no transmit light at that frequency. When a range of values is reported, it is the highest and lowest values observed within the material's light transmission range.
Refractive index values can be measured for both solids and liquids. ASTM testing standards for solids include ASTM D542 for plastics, ASTM D1747 for soft and viscous materials, and E1967 for glass. ISO standards include 489 for plastics and 18369 for contact lens materials.