the act or process of deducting; subtraction.
something that is or may be deducted: She took deductions for a home office and other business expenses from her taxes.
the act or process of deducing.
something that is deduced: His astute deduction was worthy of Sherlock Holmes.
a process of reasoning in which a conclusion follows necessarily from the premises presented, so that the conclusion cannot be false if the premises are true.
a conclusion reached by this process. Compare induction ( def 4 ).

1400–50; late Middle English deduccioun (< Anglo-French) < Latin dēductiōn- (stem of dēductiō) a leading away. See deduct, -ion

nondeduction, noun
prededuction, noun

deduction, extrapolation, induction, generalization, hypothesis. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
deduction (dɪˈdʌkʃən)
1.  the act or process of deducting or subtracting
2.  something, esp a sum of money, that is or may be deducted
3.  a.  the process of reasoning typical of mathematics and logic, whose conclusions follow necessarily from their premises
 b.  an argument of this type
 c.  the conclusion of such an argument
4.  logic
 a.  a systematic method of deriving conclusions that cannot be false when the premises are true, esp one amenable to formalization and study by the science of logic
 b.  Compare induction an argument of this type

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

late 15c., "action of deducting," from L. deductionem, noun of action from deducere (see deduce). Meaning "that which is deducted" is from 1540s.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
deduction   (dĭ-dŭk'shən)  Pronunciation Key 
  1. The process of reasoning from the general to the specific, in which a conclusion follows necessarily from the premises.

  2. A conclusion reached by this process.

Our Living Language  : The logical processes known as deduction and induction work in opposite ways. In deduction general principles are applied to specific instances. Thus, using a mathematical formula to figure the volume of air that can be contained in a gymnasium is applying deduction. Similarly, applying a law of physics to predict the outcome of an experiment is reasoning by deduction. By contrast, induction makes generalizations based on a number of specific instances. The observation of hundreds of examples in which a certain chemical kills plants might prompt the inductive conclusion that the chemical is toxic to all plants. Inductive generalizations are often revised as more examples are studied and more facts are known. If certain plants that have not been tested turn out to be unaffected by the chemical, the conclusion about the chemical's toxicity must be revised or restricted. In this way, an inductive generalization is much like a hypothesis.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary

deduction definition

A process of reasoning that moves from the general to the specific. (Compare induction.)

deduction definition

A cost or expense subtracted from revenue, usually for tax purposes.

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
Through the logical deduction of how, an understanding arises.
In addition, it will end charity car donation since the voucher would far
  exceed the tax deduction.
They are able to express logical thought, and demonstrate simple deduction at
  about age 2.
But in trying to avert a minor deduction, she would end up paying a major price.
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