The Collaborative International Dictionary of English v.0.48:

Cloister \Clois"ter\, n. [OF. cloistre, F. clo[^i]tre, L.
   claustrum, pl. claustra, bar, bolt, bounds, fr. claudere,
   clausum, to close. See Close, v. t., and cf. Claustral.]
   [1913 Webster]
   1. An inclosed place. [Obs.] --Chaucer.
      [1913 Webster]

   2. A covered passage or ambulatory on one side of a court;
      (pl.) the series of such passages on the different sides
      of any court, esp. that of a monastery or a college.
      [1913 Webster]

            But let my due feet never fail
            To walk the studious cloister's pale. --Milton.
      [1913 Webster]

   3. A monastic establishment; a place for retirement from the
      world for religious duties.
      [1913 Webster]

            Fitter for a cloister than a crown.   --Daniel.
      [1913 Webster]

   Cloister garth (Arch.), the garden or open part of a court
      inclosed by the cloisters.

   Syn: Cloister, Monastery, Nunnery, Convent, Abbey,
        Priory.

   Usage: Cloister and convent are generic terms, and denote a
          place of seclusion from the world for persons who
          devote their lives to religious purposes. They differ
          is that the distinctive idea of cloister is that of
          seclusion from the world, that of convent, community
          of living. Both terms denote houses for recluses of
          either sex. A cloister or convent for monks is called
          a monastery; for nuns, a nunnery. An abbey is a
          convent or monastic institution governed by an abbot
          or an abbess; a priory is one governed by a prior or a
          prioress, and is usually affiliated to an abbey.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cloister \Clois"ter\, v. t. [imp. & p. p. Cloistered; p. pr. &
   vb. n. Cloistering.]
   To confine in, or as in, a cloister; to seclude from the
   world; to immure.
   [1913 Webster]

         None among them are thought worthy to be styled
         religious persons but those that cloister themselves up
         in a monastery.                          --Sharp.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cloister
    n 1: residence that is a place of religious seclusion (such as a
         monastery) [syn: religious residence, cloister]
    2: a courtyard with covered walks (as in religious institutions)
    v 1: surround with a cloister, as of a garden
    2: surround with a cloister; "cloister the garden"
    3: seclude from the world in or as if in a cloister; "She
       cloistered herself in the office"

Moby Thesaurus II by Grady Ward, 1.0:

141 Moby Thesaurus words for "cloister":
   Easter sepulcher, abbey, access, adytum, aisle, alley, ambry,
   ambulatory, aperture, apse, arcade, areaway, artery, ashram,
   avenue, baptistery, blindstory, bottle up, box in, box up,
   breezeway, cabin, cage, casket, cell, chancel, channel, check,
   choir, cloisters, closet, coffin, colonnade, communication,
   conduit, confessional, confessionary, confine, connection,
   constrain, convent, coop, coop in, coop up, cork up, corridor,
   covered way, cramp, crib, crypt, defile, den, detain, diaconicon,
   diaconicum, encage, encase, enclose, entomb, exit, fence in, ferry,
   ford, friary, gallery, hall, hallway, hem in, hermitage, hideaway,
   hideout, hiding place, hold, hold in custody, hold in restraint,
   holy of holies, house, immure, impound, inhibit, inlet,
   interchange, intersection, ivory tower, junction, keep in,
   keep in custody, keep in detention, lair, lamasery, lane, loggia,
   mew, mew up, monastery, nave, nunnery, opening, outlet, overpass,
   pass, passage, passageway, pen, pen up, pergola, peristyle, porch,
   portico, pound, presbytery, priorate, priory, privacy, rail in,
   railroad tunnel, recess, restrain, restrict, retreat, rood loft,
   rood stair, rood tower, sacrarium, sacristy, sanctum,
   sanctum sanctorum, seal up, secret place, shackle, shut in,
   shut up, straiten, traject, trajet, transept, triforium, tunnel,
   underpass, vestry, wall in