-adsuff. In the direction of; toward: cephalad.[From Latin ad, to; see ad- in Indo-European roots.]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.-ad suffix forming nouns 1. a group or unit (having so many parts or members): triad. 2. (Poetry) an epic poem concerning (the subject indicated by the stem): Dunciad. [via Latin from Greek -ad- (plural -ades), originally forming adjectives; names of epic poems are all formed on the model of the Iliad]-ad suffix forming adverbs (Anatomy) denoting direction towards a specified part in anatomical descriptions: cephalad. [from Latin ad to, towards]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014ad1 (æd) n. 1. an advertisement. 2. advertising: an ad agency. [1835?45; by shortening] ad2 (æd) n. Tennis. advantage (def. 4). [1925?30; by shortening] ad- a prefix occurring in verbs or verbal derivatives borrowed from Latin, where it meant ?toward? and indicated direction, tendency, or addition: adjoin. For variants before a following consonant, see a-5, ac-, af-, ag-, al-, an-2, ap-1, ar-, as-, at-. [