1: From institutional economics: Economic institutions include norms (uncodified social rules), laws, and property relations (such as private property or communal property).
2: From sociology: Social institutions are established or standardized patterns of rule-governed behavior. They include the family, education, religion, and economic and political institutions. The also include the nested systems of relationships, processes, beliefs, practices, arrangements, expectations, and incentives that constitute these higher-order institutions.
See Topics in Sociology for more information, examples, and interesting food for thought.
3: A large organization that has a particular kind of work or purpose.
(e.g., financial/educational/research etc. institution)
...The food and Agriculture Organization (FAO), the IPCC, the World Bank, central banks in general.
Other definitions (not necessarily used in this class):
4: A building that people are sent to when they need to be looked after, for example old people or children with no parents - often used to show disapproval: I was determined not to put my mother in an institution.
5: When something is started or introduced in society, especially something relating to the law or politics: The institution of divorce proceedings
6: To be an institution: if a person, place, event, etc. is an institution, they have been an important part of a place for a very long time - often used humorously: The British pub isn't just somewhere to drink - it's an institution.