Definitions of deduction:

noun: reasoning from the general to the particular (or from cause to effect)
noun: something that is inferred (deduced or entailed or implied)
noun: an amount or percentage deducted
noun: the act of subtracting (removing a part from the whole)
noun: the act of reducing the selling price of merchandise
noun: a reduction in the gross amount on which a tax is calculated; reduces taxes by the percentage fixed for the taxpayer's income bracket