er·ror (?r??r)n.1. An act, assertion, or belief that unintentionally deviates from what is correct, right, or true.2. The condition of having incorrect or false knowledge.3. The act or an instance of deviating from an accepted code of behavior.4. A mistake.5. Mathematics The difference between a computed or measured value and a true or theoretically correct value.6. Abbr. E Baseball A defensive fielding or throwing misplay by a player when a play normally should have resulted in an out or prevented an advance by a base runner.[Middle English errour, from Old French, from Latin error, from err?re, to err; see ers- in Indo-European roots.]er?ror·less adj.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.error (??r?) n1. a mistake or inaccuracy, as in action or speech: a typing error. 2. an incorrect belief or wrong judgment3. the condition of deviating from accuracy or correctness, as in belief, action, or speech: he was in error about the train times. 4. deviation from a moral standard; wrongdoing: he saw the error of his ways. 5. (Statistics) maths statistics a measure of the difference between some quantity and an approximation to or estimate of it, often expressed as a percentage: an error of 5%. 6. (Statistics) statistics See type I error, type II error[C13: from Latin, from err?re to err] ?error-?free adjCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014er?ror (??r ?r) n. 1. a deviation from accuracy or correctness; mistake. 2. the holding of mistaken opinions. 3. the condition of believing what is not true: I was in error about the date. 4. a moral offense. 5. a baseball misplay allowing a batter to reach base or a runner to advance. 6. the difference between the observed or approximately determined value and the true value of a quantity in mathematics or statistics. 7. Law. a mistake in a matter of fact or law in a case tried in a court of record. 8. a postage stamp distinguished by an imperfection, as in design. [1250?1300; Middle English errour An error is a mistake.You can say that something is done in error. This is a fairly formal use.In conversation, you usually say that something is done by mistake.