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sub-

1.
a prefix occurring originally in loanwords from Latin (subject; subtract; subvert; subsidy); on this model, freely attached to elements of any origin and used with the meaning “under,” “below,” “beneath” (subalpine; substratum), “slightly,” “imperfectly,” “nearly” (subcolumnar; subtropical), “secondary,” “subordinate” (subcommittee; subplot).
2.
Chemistry.
  1. a prefix indicating a basic compound:
    subacetate; subcarbonate; subnitrate.
  2. a prefix indicating that the element is present in a relatively small proportion, i.e., in a low oxidation state:
    subchloride; suboxide.
Also, su-, suc-, suf-, sug-, sum-, sup-, sur-, sus-.
Origin
< Latin, combining form representing sub (preposition); akin to Greek hypó; see hypo-
Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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British Dictionary definitions for sub-

sub-

prefix
1.
situated under or beneath: subterranean
2.
secondary in rank; subordinate: subeditor
3.
falling short of; less than or imperfectly: subarctic, subhuman
4.
forming a subdivision or subordinate part of a whole: subcommittee
5.
(in chemistry)
  1. indicating that a compound contains a relatively small proportion of a specified element: suboxide
  2. indicating that a salt is basic salt: subacetate
Word Origin
from Latin sub
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 sub-

word-forming element meaning "under," from Latin preposition sub "under" (also "close to, up to, towards"), from a variant form (*(s)up-, perhaps representing *ex-upo-) of PIE root *upo- "from below," hence "turning upward, upward, up, up from under, over, beyond" (cf. Sanskrit upa "near, under, up to, on," Greek hypo "under," Gothic iup, Old Norse, Old English upp "up, upward," Hittite up-zi "rises"). Used as a prefix and in various combinations.

The original meaning is now obscured in many words from Latin ( suggest, suspect, subject, etc.). The prefix is active in Modern English, sometimes meaning "subordinate" (as in subcontinent, first recorded 1863) or "inferior" (a sense first attested 1963).

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

sub- pref.

  1. Below; under; beneath: subcutaneous.

  2. Subordinate; secondary: subinfection.

  3. Subdivision: subkingdom.

  4. Less than completely or normally; nearly; almost: subfertility.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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sub- in Science
sub-  
A prefix that means "underneath or lower" (as in subsoil), "a subordinate or secondary part of something else" (as in subphylum.), or "less than completely" (as in subtropical.)
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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5
7
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