The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   Note: Ambassadors are either ordinary [or resident] or
         extraordinary, that is, sent upon some special or
         unusual occasion or errand. --Abbott.
         [1913 Webster]

   2. An official messenger and representative.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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