Source : Webster's Revised Unabridged Dictionary (1913)

Ambassador \Am*bas"sa*dor\, Embassador \Em*bas"sa*dor\, n. [See
   {Embassador}.]
   1. A minister of the highest rank sent to a foreign court to
      represent there his sovereign or country.

   Note: Ambassadors are either ordinary [or resident] or
         extraordinary, that is, sent upon some special or
         unusual occasion or errand. --Abbott.

   2. An official messenger and representative.

Source : WordNet®

ambassador
     n 1: a diplomat of the highest rank; accredited as representative
          from one country to another [syn: {embassador}]
     2: an informal representative; "an ambassador of good will"