Dictionary.com
definitions

hemi-

  1. a combining form meaning “half,” used in the formation of compound words: hemimorphic.

Origin of hemi-

< Greek hēmi- half; cognate with Latin sēmi- semi-
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for hemi-

hemi-

prefix
  1. halfhemicycle; hemisphere Compare demi- (def. 1), semi- (def. 1)

Word Origin for hemi-

from Latin, from 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 hemi-

word-forming element meaning "half," from Greek hemi- "half," from PIE root *semi-, which is the source of Sanskrit sami, Latin semi- (see semi-), Old High German sami- "half," and Old English sam-, denoting a partial or imperfect condition (see sandblind).

Online Etymology Dictionary, © 2010 Douglas Harper
hemi- in Medicine

hemi-

pref.
  1. Half:hemicardia.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
hemi- in Science

hemi-

  1. A prefix meaning “half,” as in hemisphere, half a sphere.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.