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[sem-ee, sem-ahy]
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noun Informal.
  1. semitrailer(def 1).
  2. Often semis. semifinal(def 3).
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Origin of semi

by shortening


  1. a combining form borrowed from Latin, meaning “half,” freely prefixed to English words of any origin, now sometimes with the senses “partially,” “incompletely,” “somewhat”: semiautomatic; semidetached; semimonthly; semisophisticated.
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Origin of semi-

Middle English < Latin sēmi-; cognate with Old English sōm-, sām- half (modern dial. sam-), Old High German sāmi-, Sanskrit sāmi-, Greek hēmi-; cf. sesqui-

Usage note

See bi-1.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Related Words


Examples from the Web for semi

Contemporary Examples

Historical Examples

  • Clearly these should only be planted in wild and semi–wild places.

    The Wild Garden

    William Robinson

  • Semi has had more success than I require, and considerably more than I expected.

  • Semi can give him the former, and over the latter our future journey lies.

    Buffalo Land

    W. E. Webb

  • Raising his hand in a semi salute, the man started into the woods.

  • It continues to be the resort of persons of every civilized, and almost every semi civilized, nation on the face of the earth.

British Dictionary definitions for semi


noun plural semis
  1. British a semidetached house
  2. short for semifinal
  3. US, Canadian, Australian and NZ short for semitrailer
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  1. halfsemicircle Compare demi- (def. 1), hemi-
  2. partially, partly, not completely, or almostsemiprofessional; semifinal
  3. occurring twice in a specified period of timesemiannual; semiweekly
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Word Origin

from Latin; compare Old English sōm-, sām- half, Greek hēmi-
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 semi


before vowels sem-, word-forming element meaning "half, part, partly; partial, imperfect; twice," from Latin semi- "half," from PIE *semi- "half" (cf. Sanskrit sami "half," Greek hemi- "half," Old English sam-, Gothic sami- "half").

Old English cognate sam- was used in such compounds as samhal "poor health," literally "half-whole;" samsoden "half-cooked," figuratively "stupid" (cf. half-baked); samcucu "half-dead," literally "half-alive;" and the last survivor of the group, sandblind "dim-sighted" (q.v.). Common in Latin (e.g. semi-gravis "half-drunk," semi-hora "half hour," semi-mortuus "half-dead," semi-nudus "half-naked," semi-vir "half-man, hermaphrodite"). The Latin-derived form in English has been active in forming native words since 15c.

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Online Etymology Dictionary, © 2010 Douglas Harper

semi in Medicine


  1. Half:semicanal.
  2. Partial; partially:semiconscious.
  3. Resembling or having some of the characteristics of:semilunar.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

semi in Science


  1. A prefix that means “half,” (as in semicircle, half a circle) or “partly, somewhat, less than fully,” (as in semiconscious, partly conscious).
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.