What is the meaning of Ratio?

The relation which one quantity or magnitude has to another of the same kind. It is expressed by the quotient of the division of the first by the second; thus, the ratio of 3 to 6 is expressed by / or /; of a to b by a/b; or (less commonly) the second term is made the dividend; as, a:b = b/a.

Source: Webster’s Revised Unabridged Dictionary (1913), via OPTED