The Collaborative International Dictionary of English v.0.48:

Dad \Dad\ (d[a^]d), n. [Prob. of Celtic origin; cf. Ir. daid,
   Gael. daidein, W. tad, OL. tata, Gr. ta`ta, te`tta, Skr.
   t[=a]ta.]
   Father; -- a word sometimes used by children.
   [1913 Webster]

         I was never so bethumped with words,
         Since I first called my brother's father dad. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dad
    n 1: an informal term for a father; probably derived from baby
         talk [syn: dad, dada, daddy, pa, papa, pappa,
         pop]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "dad":
   Methuselah, abba, antediluvian, antique, back number, conservative,
   daddy, dodo, elder, father, fogy, fossil, foster father, fud,
   fuddy-duddy, genitor, governor, granny, has-been, longhair,
   matriarch, mid-Victorian, mossback, old believer, old crock,
   old dodo, old fogy, old liner, old man, old poop, old woman,
   old-timer, pa, pap, papa, pappy, pater, paterfamilias, patriarch,
   pop, pops, reactionary, regular old fogy, relic, sire, square,
   starets, stepfather, the old man, traditionalist



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

DAD
       Desktop Application Director (WordPerfect)


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

DAD
       Duplicate Address Detection (NDP, IPv6)