Balance or balancedness is a property of apportionment methods, which are methods of allocating identical items between among agens, such as dividing seats in a parliament among political parties or federal states. The property says that, if two agents have exactly the same entitlements, then the number of items they receive should differ by at most one. So if two parties win the same number of votes, or two states have the same populations, then the number of seats they receive should differ by at most one.
Ideally, agents with identical entitlements should receive an identical number of items, but this may be impossible due to the indivisibility of the items. Balancedness requires that the difference between identical-entitlement agents should be the smallest difference allowed by the indivisibility, which is 1. For example, if there are 2 equal-entitlement agents and 9 items, then the allocations (4,5) and (5,4) are both allowed, but the allocations (3,6) or (6,3) are not - a difference of 3 is not justified even by indivisibility.
Definitions.
There is a "resource" to allocate, denoted by formula_1. For example, it can be an integer representing the number of seats in a "h"ouse of representatives. The resource should be allocated between some formula_2 "agents", such as states or parties. The agents have different "entitlements", denoted by a vector formula_3. For example, "ti" can be the fraction of votes won by party "i". An "allocation" is a vector formula_4 with formula_5. An "allocation rule" is a rule that, for any formula_1 and entitlement vector formula_3, returns an allocation vector formula_4.
An allocation rule is called balanced if formula_9 implies formula_10 for all "i,j". Equivalently, formula_9 implies formula_12 for all "i,j". 
Properties.
All known apportionment methods are balanced. In particular, both Highest averages methods and Largest remainder methods are balanced.
Every apportionment method that is anonymous, exact and coherent, is also balanced. 