Source : Webster's Revised Unabridged Dictionary (1913)

Excellency \Ex"cel*len*cy\, n.; pl. {Excellencies}.
   1. Excellence; virtue; dignity; worth; superiority.

            His excellency is over Israel.        --Ps. lxviii.
                                                  34.

            Extinguish in men the sense of their own excellency.
                                                  --Hooker.

   2. A title of honor given to certain high dignitaries, esp.
      to viceroys, ministers, and ambassadors, to English
      colonial governors, etc. It was formerly sometimes given
      to kings and princes.

Source : WordNet®

Excellency
     n : a title used to address dignitaries (such as ambassadors or
         governors); usually preceded by Your or His or Her; "Your
         Excellency"