What does "alligation" mean?

Alligation: (Arith.) A rule relating to the solution of questions concerning the compounding or mixing of different ingredients, or ingredients of different qualities or values. Note: The rule is named from the method of connecting together the terms by certain ligature-like signs. Alligation is of two kinds, medial and alternate; medial teaching the method of finding the price or quality of a mixture of several simple ingredients whose prices and qualities are known; alternate, teaching the amount of each of several simple ingredients whose prices or qualities are known, which will be required to make a mixture of given price or quality.