Source : Webster's Revised Unabridged Dictionary (1913)

Authoritative \Au*thor"i*ta*tive\, a.
   1. Having, or proceeding from, due authority; entitled to
      obedience, credit, or acceptance; determinate; commanding.

            The sacred functions of authoritative teaching.
                                                  --Barrow.

   2. Having an air of authority; positive; dictatorial;
      peremptory; as, an authoritative tone.

            The mock authoritative manner of the one, and the
            insipid mirth of the other.           --Swift.
      -- {Au*thor"i*ta*tive*ly}, adv. --
      {Au*thor"i*ta*tive*ness}, n.

Source : WordNet®

authoritative
     adj 1: having authority or ascendancy or influence; "an important
            official"; "the captain's authoritative manner" [syn:
            {important}]
     2: of recognized authority or excellence; "the definitive work
        on Greece"; "classical methods of navigation" [syn: {classical},
         {definitive}]
     3: sanctioned by established authority; "an authoritative
        communique"; "the authorized biography" [syn: {authorized},
         {authorised}]