an·no Dom·i·ni (?n?? d?m??-n??, -n??)adv. Abbr. AD or ad In a specified year of the Christian era.[Medieval Latin ann? Domin? : Latin ann?, ablative of annus, year + Latin Domin?, genitive of Dominus, Lord.]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.anno Domini (?æn?? ?d?m??na?; -?ni?) adv the full form of ADninformal advancing old age[Latin: in the year of our Lord]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-. [