semi

noun Informal.

Often, semis. semifinal ( def 3 ).

Origin:
by shortening

Dictionary.com Unabridged

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.

Origin:
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 Dictionary, © Random House, Inc. 2013.
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00:10

Semi is always a great word to know.

So is callithumpian. Does it mean:

a children's mummer's parade, as on the Fourth of July, with prizes for the best costumes.

a chattering or flighty, light-headed person.

LEARN MORE UNUSUAL WORDS WITH WORD DYNAMO...

Collins

World English Dictionary

semi (ˈsɛmɪ)

—n , pl semis
1.	(Brit) a semidetached house
2.	short for semifinal
3.	(US), (Canadian), (Austral), (NZ) short for semitrailer

semi-

—*prefix*
1.	demi- Compare hemi- half: semicircle
2.	partially, partly, not completely, or almost: semiprofessional; semifinal
3.	occurring twice in a specified period of time: semiannual; semiweekly

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

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Etymonline

Word Origin & History

semi-

from L. semi- "half," from PIE *semi- (cf. Skt. sami "half," Gk. hemi- "half," O.E. sam-, Goth. sami- "half"). O.E. cognate sam- was used in such compounds as samhal "poor health," lit. "half-whole;" samsoden "half-cooked," figuratively "stupid" (cf. half-baked); samcucu "half-dead," lit. "half-alive;"

and the last survivor of the group, sand-blind "dim-sighted" (q.v.). The Latin-derived form has been common in forming native words since 15c.

Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage

Medical Dictionary

semi- pref.

Half: semicanal.
Partial; partially: semiconscious.
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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American Heritage

Science Dictionary

semi-
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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Slang Dictionary

semi

/se'mee/ or /se'mi:/
1. n. Abbreviation for `semicolon', when speaking. "Commands to grind are prefixed by semi-semi-star" means that the prefix is `;;*', not 1/4 of a star.
2. A prefix used with words such as `immediately' as a qualifier. "When is the system coming up?" "Semi-immediately." (That is, maybe not for an hour.) "We did consider that possibility semi-seriously." See also infinite.

Jargon
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FOLDOC

Computing Dictionary

semi definition

/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]
(1995-01-31)

American Heritage

Abbreviations & Acronyms

semi