An apportionment is an allocation based on some proportions.
I associate the term apportionment with a corporation's taxable income that was earned in many states within the U.S. In that situation, the corporation's taxable income must be allocated or apportioned to each of the states based upon certain accepted factors. In the past, the apportionment or allocation was often based on a corporation's tangible property, employees, and sales in each of the states. More recently, apportionments seem to be based more on sales or receipts within each state.
Since AccountingCoach.com does not cover income taxes, you should contact a tax professional for a more accurate and complete explanation.