The Collaborative International Dictionary of English v.0.48:

Keeper \Keep"er\, n.
   1. One who, or that which, keeps; one who, or that which,
      holds or has possession of anything.
      [1913 Webster]

   2. One who retains in custody; one who has the care of a
      prison and the charge of prisoners.
      [1913 Webster]

   3. One who has the care, custody, or superintendence of
      anything; as, the keeper of a park, a pound, of sheep, of
      a gate, etc.; the keeper of attached property; hence, one
      who saves from harm; a defender; a preserver.
      [1913 Webster]

            The Lord is thy keeper.               --Ps. cxxi. 6.
      [1913 Webster]

   4. One who remains or keeps in a place or position.
      [1913 Webster]

            Discreet; chaste; keepers at home.    --Titus ii. 5.
      [1913 Webster]

   5. A ring, strap, clamp, or any device for holding an object
      in place; as:
      (a) The box on a door jamb into which the bolt of a lock
          protrudes, when shot.
      (b) A ring serving to keep another ring on the finger.
      (c) A loop near the buckle of a strap to receive the end
          of the strap.
          [1913 Webster]

   6. A fruit that keeps well; as, the Roxbury Russet is a good
      keeper. Hence: Anything perishable that remains in good
      condition longer than usual. -- Downing.
      [1913 Webster +PJC]

   7. An iron bar that is placed on the poles of a horseshoe
      magnet, and held in place there by the magnetic force, to
      preserve the strength of the magnet when not in use.
      [PJC]

   Keeper of the forest (O. Eng. Law), an officer who had the
      principal government of all things relating to the forest.


   Keeper of the great seal, a high officer of state, who has
      custody of the great seal. The office is now united with
      that of lord chancellor. [Eng.]

   Keeper of the King's conscience, the lord chancellor; -- a
      name given when the chancellor was an ecclesiastic. [Eng.]


   Keeper of the privy seal (styled also lord privy seal), a
      high officer of state, through whose hands pass all
      charters, pardons, etc., before they come to the great
      seal. He is a privy councillor, and was formerly called
      clerk of the privy seal. [Eng.]

   Keeper of a magnet, a piece of iron which connects the two
      poles, for the purpose of keeping the magnetic power
      undiminished; an armature; called also keeper.
      [1913 Webster]

WordNet (r) 3.0 (2006):

keeper
    n 1: someone in charge of other people; "am I my brother's
         keeper?"
    2: one having charge of buildings or grounds or animals [syn:
       custodian, keeper, steward]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "keeper":
   Audubon Society, National Wildlife Service, Sierra Club,
   air warden, attendant, bull, caretaker, castellan, cerberus,
   commandant, conservationist, conservator, curator, custodian,
   custos, deliverer, enjoyer, forest ranger, game warden, gamekeeper,
   gaoler, governor, guard, guardian, guardian angel, have, haver,
   holder, jailer, janitor, lifeguard, lifesaver, next friend, nurse,
   possessor, preservationist, preserver, principal keeper,
   prison guard, prochein ami, ranger, rescuer, safekeeper, saver,
   savior, screw, shepherd, steward, taxidermist, turnkey, warden,
   warder, watchdog