The Collaborative International Dictionary of English v.0.48:

Institution \In`sti*tu"tion\, n. [L. institutio: cf. F.
   institution.]
   [1913 Webster]
   1. The act or process of instituting; as:
      (a) Establishment; foundation; enactment; as, the
          institution of a school.
          [1913 Webster]

                The institution of God's law is described as
                being established by solemn injunction.
                                                  --Hooker.
      (b) Instruction; education. [Obs.] --Bentley.
      (c) (Eccl. Law) The act or ceremony of investing a
          clergyman with the spiritual part of a benefice, by
          which the care of souls is committed to his charge.
          --Blackstone.
          [1913 Webster]

   2. That which instituted or established; as:
      (a) Established order, method, or custom; enactment;
          ordinance; permanent form of law or polity.
          [1913 Webster]

                The nature of our people,
                Our city's institutions.          --Shak.
      (b) An established or organized society or corporation; an
          establishment, especially of a public character, or
          affecting a community; a foundation; as, a literary
          institution; a charitable institution; also, a
          building or the buildings occupied or used by such
          organization; as, the Smithsonian Institution.
      (c) Anything forming a characteristic and persistent
          feature in social or national life or habits.
          [1913 Webster]

                We ordered a lunch (the most delightful of
                English institutions, next to dinner) to be
                ready against our return.         --Hawthorne.
          [1913 Webster]

   3. That which institutes or instructs; a textbook; a system
      of elements or rules; an institute. [Obs.]
      [1913 Webster]

            There is another manuscript, of above three hundred
            years old, . . . being an institution of physic.
                                                  --Evelyn.
      [1913 Webster]

WordNet (r) 3.0 (2006):

institution
    n 1: an organization founded and united for a specific purpose
         [syn: institution, establishment]
    2: an establishment consisting of a building or complex of
       buildings where an organization for the promotion of some
       cause is situated
    3: a custom that for a long time has been an important feature
       of some group or society; "the institution of marriage"; "the
       institution of slavery"; "he had become an institution in the
       theater"
    4: the act of starting something for the first time; introducing
       something new; "she looked forward to her initiation as an
       adult"; "the foundation of a new scientific society" [syn:
       initiation, founding, foundation, institution,
       origination, creation, innovation, introduction,
       instauration]
    5: a hospital for mentally incompetent or unbalanced person
       [syn: mental hospital, psychiatric hospital, mental
       institution, institution, mental home, insane asylum,
       asylum]

Moby Thesaurus II by Grady Ward, 1.0:

158 Moby Thesaurus words for "institution":
   A, academy, act, agency, alpha, apostolic orders, appointment,
   asylum, atelier, barbershop, beauty parlor, beauty shop, beginning,
   bench, bill, blast-off, butcher shop, bylaw, calling, canon,
   canonization, ceremonial, ceremony, college, commencement, company,
   concern, conferment, consecration, constitution, corporation,
   creation, custom, cutting edge, dawn, decree, desk, dictate,
   dictation, doctrine, dogma, duty, edge, edict, effectuation,
   election, enactment, establishing, establishment, facility, firm,
   fixture, flying start, form, form of worship, formality, formation,
   forming, formula, formulary, foundation, founding, fresh start,
   function, habit, holy orders, holy rite, home, hospital, house,
   inauguration, inception, induction, installation, institute,
   introduction, investiture, jump-off, jus, kick-off, law,
   leading edge, legislation, lex, liturgy, loft, major orders,
   materialization, measure, minor orders, mode of worship, mystery,
   new departure, nomination, observance, office, oncoming, onset,
   opening, ordainment, order of worship, orders, ordinance,
   ordination, ordonnance, organization, origin, origination,
   outbreak, outset, parlor, practice, preferment, prescribed form,
   prescript, prescription, presentation, reading in, realization,
   regulation, rite, ritual, ritual observance, rituality, routine,
   rubric, rule, ruling, running start, sacrament, sacramental,
   school, send-off, service, setting in motion, setting-up, shop,
   solemnity, square one, standing order, start, start-off,
   starting point, statute, studio, sweatshop, take-off, tradition,
   university, work site, work space, workbench, workhouse,
   working space, workplace, workroom, workshop, worktable



Bouvier's Law Dictionary, Revised 6th Ed (1856):

INSTITUTION, eccl. law. The act by which the ordinary commits the cure of
souls to a person presented to a benefice.



Bouvier's Law Dictionary, Revised 6th Ed (1856):

INSTITUTION, political law. That which has been established and settled by
law for the public good; as, the American institutions guaranty to the
citizens all privileges and immunities essential to freedom.



Bouvier's Law Dictionary, Revised 6th Ed (1856):

INSTITUTION, practice. The commencement of an action; as, A B has instituted
a suit against C D, to recover damages for a trespass.