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-ad1

  1. a suffix occurring in loanwords from Greek denoting a group or unit comprising a certain number, sometimes of years: dyad; triad.
  2. a suffix meaning “derived from,” “related to,” “concerned with,” “associated with” (oread), introduced in loanwords from Greek (Olympiad; oread), used sporadically in imitation of Greek models, as Dunciad, after Iliad.
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Origin of -ad1

Greek -ad- (stem of -as), specialization of feminine adjective-forming suffix, often used substantively

-ad2

  1. variant of -ade1: ballad.
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-ad3

  1. Anatomy, Zoology. a suffix forming adverbs from nouns signifying parts of the body, denoting a direction toward that part: dextrad; dorsad; mediad.
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Origin of -ad3

From the Latin word ad toward, anomalously suffixed to the noun; introduced as a suffix by Scottish anatomist John Barclay (1758–1826) in 1803
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for -ad

-ad1

suffix forming nouns
  1. a group or unit (having so many parts or members)triad
  2. an epic poem concerning (the subject indicated by the stem)Dunciad
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Word Origin

via Latin from Greek -ad- (plural -ades), originally forming adjectives; names of epic poems are all formed on the model of the Iliad

-ad2

suffix forming adverbs
  1. denoting direction towards a specified part in anatomical descriptionscephalad
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Word Origin

from Latin ad to, towards
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 -ad

word-forming element denoting collective numerals (cf. Olympiad), plant families, and names of poems, from Greek -as (genitive -ados), a suffix forming fem. nouns; also used in fem. patronymics (Dryad, Naiad, also, in plural, Pleiades, Hyades).

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

-ad

suff.
  1. In the direction of; toward:cephalad.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.