Etymology
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deduction (n.)

early 15c., deduccioun, "a bringing, a leading;" mid-15c., "action of deducting; a taking away, a number or amount subtracted," from Old French deduction (Modern French déduction) and directly from Latin deductionem (nominative deductio) "a leading away, an escorting; a diminution," noun of action from past-participle stem of deducere "lead or bring away or down; derive" (in Medieval Latin, "infer logically"), from de "down" (see de-) + ducere "to lead," from PIE root *deuk- "to lead."

Meaning "that which is deducted" is from 1540s. As a term in logic, "derivation as a result from a known principle, an inference, conclusion," 1520s, from Late Latin use of deductio as a loan-translation of Greek apagoge. Related: Deductional.

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*deuk- 

Proto-Indo-European root meaning "to lead."

It forms all or part of: abduce; abducent; abduct; abduction; adduce; aqueduct; circumduction; conduce; conducive; conduct; conductor; conduit; deduce; deduction; dock (n.1) "ship's berth;" doge; douche; ducal; ducat; Duce; duchess; duchy; duct; ductile; duke (n.); educate; education; induce; induction; introduce; introduction; misconduct; produce; production; reduce; reduction; seduce; seduction; subduce; subduction; taut; team (n.); teem (v.1) "abound, swarm, be prolific;" tie (n.); tow (v.); traduce; transducer; tug; zugzwang.

It is the hypothetical source of/evidence for its existence is provided by: Latin dux (genitive ducis) "leader, commander," in Late Latin "governor of a province," ducere "to lead;" Old English togian "to pull, drag," teonteon "to pull, drag;" German Zaum "bridle," ziehen "to draw, pull, drag;" Middle Welsh dygaf "I draw."

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stoppage (n.)

mid-15c., "deduction from payment," from stop (v.) + -age. From late 15c. as "impediment, hindrance, obstruction;" 1650s as "act of stopping."

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discount (v.)

1620s, "reckon as an abatement or deduction" (a sense now obsolete), from Old French desconter "reckon off, account back" (13c., Modern French décompter), from Medieval Latin discomputare, from dis- "away, from" (see dis-) + computare "to reckon, to count" (see compute). Hence, "to abate, deduct" (1650s), and figurative sense "to leave out of account, disregard" (1702). Formerly also discompt. Commercial sense of "make a deduction from, put a reduced price upon" is by 1977. Related: Discounted; discounting.

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rebate (n.)

1650s, "an allowance by way of discount, deduction from a sum of money to be paid," from rebate (v.). By 1882 as "a repayment, money paid back."

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discount (n.)

1620s, "abatement" (a sense now obsolete), alteration of French descompte (16c., Modern French décompte), from Medieval Latin discomputus (source of Italian disconto), from discomputare, from dis- (see dis-) + computare "to count" (see compute). Commercial meaning "deduction for early or prompt payment" is from 1680s; meaning "a reduction in the price of goods" attested by 1837.

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tax (n.)

early 14c., "obligatory contribution levied by a sovereign or government," from Anglo-French tax, Old French taxe, and directly from Medieval Latin taxa, from Latin taxare (see tax (v.)). Related: Taxes. Tax-deduction is from 1942; tax-shelter is attested from 1961.

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deductive (adj.)

1640s, "derivative" (a sense now obsolete); from 1660s in logic, "consisting of deduction; based on inference from accepted principles," from Latin deductivus"derivative," from deduct-, past-participle stem of deducere "to lead down, derive" (in Medieval Latin, "infer logically"), from de "down" (see de-) + ducere "to lead" (from PIE root *deuk- "to lead"). Related: Deductively.

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reprise (n.)

late 14c., "yearly deduction from charges upon a manor or estate," from Old French reprise "act of taking back" (13c.), fem. of repris, past participle of reprendre "take back," from Latin reprendere"pull back, hold back" (see reprehend). The meaning "resumption of an action" is attested from 1680s. The musical sense of "a repeated passage, act of repeating a passage" is by 1879.

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conclusion (n.)

late 14c., "deduction or inference reached by reasoning, result of a discussion or examination," from Old French conclusion "conclusion, result, outcome," from Latin conclusionem (nominative conclusio), noun of action from past-participle stem of concludere "to shut up, enclose" (see conclude).

Also, from late 14c. "the end, termination, final part; closing passages of a speech or writing; final result, outcome." For foregone conclusion, see forego.

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