An allotment is an amount of funds allocated for expenditure in a particular period of time, generally a fiscal quarter.
The purpose of an allotment is to ensure that funds are available to cover desired expenditures, and that said expenditures are made at the appropriate time. For example, a school district might not allot funds for renovation of the high school until summer, when students are out of school and the work can be done in time for the following school year.
The term "allotment" may also refer to a specific direction by an employee to have a portion of his/her paycheck deposited into a separate account (such as into a retirement fund).