What is the difference between ratio and proportion?

In mathematics, a ratio illustrates the relationship between two things, often quantities, while a proportion refers to the equality of two given ratios. A ratio is generally only an expression, while a proportion can be written as an algebraic equation.

When quantities having the same units are expressed as a ratio, the result is an abstract number taken from dividing one quantity by the other. A ratio can be written in fractional form or using the ":" symbol. Quantities with dissimilar units cannot be represented as a ratio. In mathematical problems involving proportions written in fractions, a missing value can be solved if all other terms are given.