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From Latin decimare ‘to take the tenth (decimus) part of anything,’ in particular referring to the levying and payment of tithe and also the practice of capital punishment applied to one man at random (by lot) out of every ten in a legion.
Third person singular
- (Roman history) To kill one man chosen by lot out of every ten in a legion or other military group.
- ca. 1650: Jeremy Taylor, (from vol. 1 of an 1835 edition of his works) - God sometimes decimates or tithes delinquent persons, and they died for a common crime, according as God hath cast their lot in the decrees of predestination.
- 1989: Said to have been martyred as a Christian legionary commander of late Roman times—for having refused an imperial order to kill one in ten (that is, decimate in the Roman meaning of the word) of the soldiers of another legion which had gone into revolt [...] —Basil Davidson, The Ancient World and Africa, in Egypt Revisited, ed. Ivan Van Sertima 
- 1998: [...] where Caesar threatened to disband Legio X after a mutiny. The men begged him to decimate them instead, and Caesar relented in the same way that Titus refrained from executing this cavalryman after his comrades’ appeal. —Adrian Goldsworthy, The Roman Army at War 
- 2004: He then declared that he would decimate Legio IX, but allowed himself to be ‘persuaded’ by the pleas of officers and men only to execute twelve of the 120 soldiers seen as ringleaders. —Adrian Goldsworthy, In the Name of Rome 
- To reduce anything by one in ten, or ten percent.
- 1996: don ~; (d) the act of removing one-tenth of a group, decimating. —Middle English Dictionary, ed. Robert E Lewis, part T.6 
- 1840: Out of nine hundred, ninety will be ejected, that the production of the others may be increased one-tenth. [...] there will be eight hundred and ten laborers producing as nine hundred, while, to accomplish their purpose, they would have to produce as one thousand. [...] Here, then, we have a society which is continually decimating itself [...] —P J Proudhon, What Is Property?  
- (historical) To exact a tithe, or tax of 10 percent
- 1669: John Dryden, The wild gallant - I have heard you are as poor as a decimated Cavalier [referring to Cromwell's ten per cent. income-tax on Cavaliers], and had not one foot of land in all the world.
- 1819: In addition, an ordinance was published that “all who had ever borne arms for the king, or declared themselves to be of the royal party, should be decimated, that is, pay a tenth part of all the estate which they had left, to support the charge which the commonwealth was put to [...] —John Lingard, History of England  
- To reduce to one-tenth.
- 1998: In this dramatic picture, the nation is literally decimated, and even the tenth which remains is subjected to a further destruction. —Israel, the Land and the People, ed. H Wayne House 
- 2000: [...] comments about the Rangers’ decimated pitching staff. Jay commented to the other drunks that although the word decimated is often used to mean “demolished” or “destroyed,” it literally means “reduced to one-tenth of its former number.” —Louise Redd, Hangover Soup 
- 2003: African slaves were needed to replace Native American populations that had been decimated (literally reduced to one-tenth their size) by European conquest. —Susan S. Hunter, Black Death 
- 1788: Yet such population [viz. 300,000 males slain] is incredible; and the second or third city of Italy need not repine if we only decimate the numbers of the present text Both Milan and Genoa revived in less than thirty years. —Edward Gibbon, History of the Decline and Fall vol. 4  
- 2005: In the New World, European colonists initially enslaved Native Americans, decimating the indigenous populations to one-tenth of their original sizes. —Wilma A. Dunaway, Put in Master’s Pocket, in Appalachians and Race, ed. John C Inscoe 
- To severely reduce; to destroy almost completely.
- 1856+: James Froude, History of England from the fall of Wolsey to the death of Elizabeth - It [England] had decimated itself for a question which involved no principle, and led to no result.
- (computer graphics) To replace a high-resolution model with one of lower resolution but acceptably similar appearance.
- 1999: A decimate tool allows us to obtain a more coarse-grained view of the data over the full n.-dimensional space. — Mihalisin, Timlin and Schwegler in Visualizing Multivariate Functions, Data and Distributions, collected in Readings in Information Visualization: Using Vision to Think, ISBN 1558605339, p.122
- 2001: However, many times it is more practical to decimate existing high-res models because of time, money or manpower issues. — in Inside 3Ds Max 4, edited by Kim Lee, ISBN 0735710945, p. 56
- 2004: Given this initial fine mesh, we smooth and decimate it to a desired mesh resolution. — Geremy Heitz, Torsten Rohlfing and Calvin Maurer in Automatic Generation of Shape Models using Nonrigid Registration with a Single Segmented Template Mesh collected in Vision Modeling and Visualization 2004 ISBN 1586034723, p. 74
The definition reduce by one in ten is occasionally cited as "the correct" definition, with severely reduce considered a "misconception," reading decimate as to reduce to one-tenth rather than by one-tenth.
The Cambridge Guide to English Usage states that the nonspecific use of this word to mean devastate or severely reduce the numbers of is ‘nowadays the commonest use of the word in both British and American English, and it’s registered without comment in modern dictionaries.’ It also advises against using numbers with the term, as ‘They are redundant where it means “reduce by one tenth,” and where it doesn't they confound the arithmetic.’ 
The 23 occurrences of decimate in the British National Corpus — compare decimates, decimated, and decimating — almost all clearly accord with the nonspecific sense. The only references to the historical sense are two complaints about modern usage and its critics. Neither of these actually uses the term to mean "reduce by one-tenth".
- “decimate” in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913.
- The Century Dictionary, The Century Co., New York, 1914
Elsewhere on the web