Euclid Elements   Book 5

Definition 1
A magnitude is a part of a magnitude, the less of the greater, when it measures the greater.
Definition 2
The greater is a multiple of the less when it is measured by the less.

Definition 3
A ratio is a sort of relation in respect of size between two magnitudes of the same kind.
Definition 4
Magnitudes are said to have a ratio to one another which can, when multiplied, exceed one another.

Definition 5
Magnitudes are said to be in the same ratio, the first to the second and the third to the fourth, when, if any equimultiples whatever are taken of the first and third, and any equimultiples whatever of the second and fourth, the former equimultiples alike exceed, are alike equal to, or alike fall short of, the latter equimultiples respectively taken in corresponding order.

Definition 6

Let magnitudes which have the same ratio be called proportional.