The Collaborative International Dictionary of English v.0.48:

Ad- \Ad-\ [A Latin preposition, signifying to. See At.]
   As a prefix ad- assumes the forms ac-, af-, ag-, al-, an-,
   ap-, ar-, as-, at-, assimilating the d with the first letter
   of the word to which ad- is prefixed. It remains unchanged
   before vowels, and before d, h, j, m, v. Examples: adduce,
   adhere, adjacent, admit, advent, accord, affect, aggregate,
   allude, annex, appear, etc. It becomes ac- before qu, as in
   acquiesce.
   [1913 Webster]

WordNet (r) 3.0 (2006):

A.D.
    adv 1: in the Christian era; used before dates after the
           supposed year Christ was born; "in AD 200" [syn: AD,
           A.D., anno Domini]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

AD
       Active Directory (MS, Windows, AD, DS)


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

AD
       Authorized Distributor (DEC)


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

AD
       Analog-to-Digital (D/A), "A/D"


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

AD
       Autonomous Domain