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 of semi-
< Latin sēmi-;
cognate with Old English sōm-, sām-
half (modern dial. sam-
), Old High German sāmi-, Sanskrit sāmi-, Greek hēmi-;
Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for semi-
partially, partly, not completely, or almostsemiprofessional; semifinal
occurring twice in a specified period of timesemiannual; semiweekly
Word Origin for semi-
from Latin; compare Old English sōm-, sām- half, Greek hēmi-
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
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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
Medicine definitions for semi-
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.
Science definitions for 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 © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.