The Collaborative International Dictionary of English v.0.48:

Guardian \Guard"i*an\ (g[aum]rd"[i^]*an or g[aum]rd"yan; 106),
   a.
   Performing, or appropriate to, the office of a protector; as,
   a guardian care.
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

guardian angel
    n 1: an angel believed to have special affection for a
         particular individual [syn: guardian spirit, guardian
         angel]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "guardian angel":
   air warden, ancestral spirits, angel, attendant, attendant godling,
   caretaker, castellan, conservator, control, curator, custodian,
   daemon, demon, fairy godmother, familiar, familiar spirit,
   forest ranger, game warden, gamekeeper, genius, genius domus,
   genius loci, good angel, good genius, governor, guardian,
   guardian spirit, guide, household gods, invisible helper, janitor,
   keeper, lares and penates, lares compitales, lares familiaris,
   lares permarini, lares praestites, lares viales, lifeguard,
   lifesaver, manes, ministering angel, next friend, numen, penates,
   prochein ami, ranger, shepherd, special providence, steward, totem,
   tutelar god, tutelary, warden, warder