a prefix appearing in loanwords from Greek, where it meant “under” (hypostasis ); on this model used, especially as opposed to hyper-, in the formation of compound words (hypothyroid ).
Also, especially before a vowel, hyp-.

< Greek, combining form of hypó under (preposition), below (adv.); cognate with Latin sub (see sub-); cf. up

hyper-, hypo-.
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World English Dictionary
hypo- or (before a vowel) hyp-
1.  under, beneath, or below: hypodermic
2.  lower; at a lower point: hypogastrium
3.  less than: hypoploid
4.  (in medicine) denoting a deficiency or an abnormally low level: hypothyroid; hypoglycaemia
5.  incomplete or partial: hypoplasia
6.  indicating that a chemical compound contains an element in a lower oxidation state than usual: hypochlorous acid
[from Greek, from hupo under]
hyp- or (before a vowel) hyp-
[from Greek, from hupo under]

indicating a plagal mode in music: Hypodorian
[from Greek: beneath (it lies a fourth below the corresponding authentic mode)]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

Gk. hypo (prep. and adv.) "under" (see sub-).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

hypo- or hyp-

  1. Below; beneath; under: hypochondriac.

  2. Less than normal; deficient: hypofunction.

  3. In the lowest state of oxidation: hypoxanthine.

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
hypo- or hyp-  
A prefix that means "beneath" or "below," as in hypodermic, below the skin. It also means "less than normal," especially in medical terms like hypoglycemia. In the names of chemical compounds, it means "at the lowest state of oxidation," as in sodium hypochlorite.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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