Optical density is a measure of the degree to which a material absorbs or scatters light. Other terms commonly used to describe this property include absorbance, transmittance, and refractive index. Absorbance measures the amount of light absorbed by a material per unit thickness, while transmittance measures the amount of light that passes through a material. Refractive index refers to the degree to which a material refracts, or bends, light. Other synonyms for optical density include optical thickness, extinction coefficient, and optical absorption. These terms are often used interchangeably in scientific literature and can be confusing for those who are unfamiliar with the field of optics.