The Collaborative International Dictionary of English v.0.48:

Guardian \Guard"i*an\, n. [OF. guardain, gardien, F. gardien,
   LL. guardianus. See Guard, v. t., and cf. Wasden.]
   1. One who guards, preserves, or secures; one to whom any
      person or thing is committed for protection, security, or
      preservation from injury; a warden.
      [1913 Webster]

   2. (Law) One who has, or is entitled to, the custody of the
      person or property of an infant, a minor without living
      parents, or a person incapable of managing his own
      affairs.
      [1913 Webster]

            Of the several species of guardians, the first are
            guardians by nature. -- viz., the father and (in
            some cases) the mother of the child.  --Blackstone.
      [1913 Webster]

   Guardian ad litem(Law), a guardian appointed by a court of
      justice to conduct a particular suit.

   Guardians of the poor, the members of a board appointed or
      elected to care for the relief of the poor within a
      township, or district.
      [1913 Webster]

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
    n 1: a person who cares for persons or property [syn:
         defender, guardian, protector, shielder]

Moby Thesaurus II by Grady Ward, 1.0:

105 Moby Thesaurus words for "guardian":
   MC, air warden, ancestral spirits, angel, attendant,
   attendant godling, attorney, bailiff, bull, butler, caretaker,
   castellan, cerberus, champion, commandant, conservator, control,
   covering, croupier, curator, custodial, custodian, custos, daemon,
   defender, defensive, demon, emcee, factor, fairy godmother,
   familiar, familiar spirit, forest ranger, fostering, game warden,
   gamekeeper, gaoler, genius, genius domus, genius loci, good angel,
   good genius, governor, guard, guardian angel, guardian spirit,
   guarding, guide, household gods, housekeeper, immunizing,
   invisible helper, jailer, janitor, keeper, landreeve,
   lares and penates, lares compitales, lares familiaris,
   lares permarini, lares praestites, lares viales, librarian,
   lifeguard, lifesaver, majordomo, manes, master of ceremonies,
   ministering angel, next friend, numen, paladin, parental, penates,
   preserver, preventive, principal keeper, prison guard,
   prochein ami, proctor, procurator, prophylactic, protecting,
   protective, protector, ranger, safeguarding, screening, screw,
   seneschal, sheltering, shepherd, shielding, special providence,
   steward, totem, trustee, turnkey, tutelar god, tutelary, vigilant,
   warden, warder, watchdog, watchful