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Used as a connective to join word elements: acidophilic.

[Middle English, from Old French, from Latin, from Greek, thematic vowel of nouns and adjectives used in combination.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.


connective vowel
used to connect elements in a compound word: chromosome; filmography. Compare -i-
[from Greek, stem vowel of many nouns and adjectives in combination]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014

O, o


n., pl. O's Os, o's os oes.
1. the 15th letter of the English alphabet, a vowel.
2. any spoken sound represented by this letter.
3. something shaped like an O.
4. a written or printed representation of the letter O or o.



interj., n., pl. O's. interj.
1. (used before a name in direct address, esp. in solemn or poetic language, to lend earnestness to an appeal): Hear, O Israel!
2. (used as an expression of surprise, pain, annoyance, longing, gladness, etc.)
3. the exclamation “O.”
[1125–75; Middle English < Old French < Latin ō]


1. Old.
2. Gram. object.


1. the 15th in order or in a series.
2. the Arabic numeral; zero; cipher.
3. a major blood group. Compare ABO system.
4. oxygen.


(ə, ō),
1. of: o'clock; will-o'-the-wisp.
2. Chiefly Dial. on.
[Middle English; by shortening.]


a prefix meaning “descendant,” in Irish family names: O'Brien; O'Connor.
[representing Irish ó descendant, Old Irish au]


, Chem.


var. of ob- before m: omission.


var. of oo-: oidium.


the typical ending of the first element of compounds of Greek origin, used regularly in forming new compounds with elements of Greek origin and often used in English as a connective irrespective of etymology: Franco-Italian; geography; seriocomic; speedometer. Compare -i-.
[Middle English (< Old French) < Latin < Greek]


1. a suffix occurring as the final element in informal shortenings of nouns (ammo; combo; promo); -o also forms nouns, usu. derogatory, for persons or things exemplifying or associated with that specified by the base noun or adjective (pinko; weirdo; wino).
2. a suffix occurring in informal noun or adjective derivatives, usu. grammatically isolated, as in address: kiddo; neato; righto.


1. pint.
[< Latin octārius]
2. October.
3. Ohio.


1. ocean.
2. pint.
[< Latin octārius]
3. octavo.
4. off.
5. old.
6. only.
7. order.
8. out.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.