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[sem-ee, sem-ahy] /ˈsɛm i, ˈsɛm aɪ/
noun, Informal.
semitrailer (def 1).
Often, semis. semifinal (def 3).
Origin of semi
by shortening


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.
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. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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Examples from the web for semi
  • The adults feed on leaves and leave semi-circular notches on the margins.
  • Even though we've had some rain, we're still in a semi-arid climate.
  • The dining room opens onto the semi-walled entrance area.
  • It is a semi-dwarf and will reach three to four feet tall and wide upon maturity.
  • It's cracked and uneven, making it semi-unsafe for anyone with mobility challenges.
  • Jack-ups and semi-submersibles are rigs used to drill for oil at sea.
  • The city has become gridlocked, with rush-hours replaced by semi-permanent congestion.
  • Mavericks and semi-celebrities catch the eye, but are not always the surest bets.
  • Whizzing across the semi-desert in an air-conditioned car is quite restful.
  • In traditional universities academics are semi-independent contractors who devote as much time as possible to their own research.
British Dictionary definitions for semi


noun (pl) semis
(Brit) a semidetached house
short for semifinal
(US & Canadian, Austral & NZ) short for semitrailer


half: semicircle Compare demi- (sense 1), hemi-
partially, partly, not completely, or almost: semiprofessional, semifinal
occurring twice in a specified period of time: semiannual, semiweekly
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
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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.

Online Etymology Dictionary, © 2010 Douglas Harper
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semi in Medicine

semi- pref.

  1. Half: semicanal.

  2. Partial; partially: semiconscious.

  3. Resembling or having some of the characteristics of: semilunar.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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semi in Science
A prefix that means "half," (as in semicircle, half a circle) or "partly, somewhat, less than fully," (as in semiconscious, partly conscious).
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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semi in Technology

/se'mee/ or /se'mi:/ A spoken abbreviation for semicolon.
"Commands to grind are prefixed by semi semi star" means that the prefix is ";;*", not 1/4 of a star.
[Jargon File]

The Free On-line Dictionary of Computing, © Denis Howe 2010
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Related Abbreviations for semi


  1. semifinal
  2. semitrailer


subendocardial myocardial infarction
The American Heritage® Abbreviations Dictionary, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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