The Collaborative International Dictionary of English v.0.48:

Anno Domini \An"no Dom"i*ni\ [L., in the year of [our] Lord
   [Jesus Christ]; usually abbrev. a. d.]
   In the year of the Christian era; as, a. d. 1887.
   [1913 Webster]

WordNet (r) 3.0 (2006):

anno Domini
    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]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

ANNO DOMINI, in the year of our Lord, abbreviated, A. D. The computation of
time from the incarnation of our Saviour which is used as the date of all
public deeds in the United States and Christian countries, on which account
it is called the "vulgar vera."