Note 1 to entry: The ratio of two quantities of the same kind is a quantity of dimension one. Examples: transfer ratio, signal-to-noise ratio, transformation ratio.

Note 2 to entry: The ratio $a/b$ is expressed by the words “ratio of a to b”.

Note 3 to entry: For the ratio of two quantities, the term “index” is sometimes used in place of ratio. Examples: refractive index, loss index.