Definitions of institution:

• noun:   an establishment consisting of a building or complex of buildings where an organization for the promotion of some cause is situated
• noun:   a custom that for a long time has been an important feature of some group or society

  Example: "The institution of marriage"

• noun:   an organization founded and united for a specific purpose
• noun:   the act of starting something for the first time; introducing something new
• noun:   a hospital for mentally incompetent or unbalanced person