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cis-

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  1. a prefix occurring in loanwords from Latin meaning “on the near side of; on this side of” (cisalpine); on this model, used in the formation of compound words (cisatlantic).
  2. Chemistry. a specialization of this denoting a geometric isomer having a pair of identical atoms or groups attached on the same side of two atoms linked by a double bond.Compare trans-(def 2).
  3. a prefix meaning “on the same side of,” referring to the alignment of one’s gender identity with one’s biological sex assigned at birth (cisgender; cissexual).
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Origin of cis-

< Latin; akin to here
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for cis-

cis-

prefix
  1. on this or the near side ofcisalpine
  2. (often in italics) indicating that two groups of atoms in an unsaturated compound lie on the same side of a double bondcis- butadiene Compare trans- (def. 5)
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Word Origin

from Latin
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for cis-

word-forming element meaning "on the near side of, on this side," from Latin preposition cis "on this side" (in reference to place or time), related to citra (adv.) "on this side," from PIE *ki-s, from root *ko- "this" (cf. Old Church Slavonic si, Lithuanian šis, Hittite ki "this," Old English hider, Gothic hidre "hither;" see he). Opposed to trans- or ultra-. Originally only of place, sometimes 19c. of time; 21c. of life situations (e.g. cis-gender, by 2011).

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Online Etymology Dictionary, © 2010 Douglas Harper

cis- in Medicine

cis-

pref.
  1. Having a pair of identical atoms or groups on the same side of a plane that passes through two carbon atoms linked by a double bond:cis-butene.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.