Source : Webster's Revised Unabridged Dictionary (1913)

Guardian \Guard"i*an\, a.
   Performing, or appropriate to, the office of a protector; as,
   a guardian care.

   {Feast of Guardian Angels} (R. C. Ch.) a church festival
      instituted by Pope Paul V., and celebrated on October 2d.
      

   {Guardian angel}.
   (a) The particular spiritual being believed in some branches
       of the Christian church to have guardianship and
       protection of each human being from birth.
   (b) Hence, a protector or defender in general. --O. W.
       Holmes.

   {Guardian spirit}, in the belief of many pagan nations, a
      spirit, often of a deceased relative or friend, that
      presides over the interests of a household, a city, or a
      region.

Source : WordNet®

guardian angel
     n : an angel believed to have special affection for a particular
         individual [syn: {guardian spirit}]