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]