a large organization that has a particular kind of work or purpose
financial/educational/research etc institution
the Government and other political institutions
powerful institutions such as world banks
the Institution of Electrical Engineers
an important system of organization in society that has existed for a long time:
social institutions such as the family and religion
the institution of marriage/monarchy etc
The scandal threatened to undermine the institution of the Presidency.
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.
a mental institution (=for the mentally ill)
when something is started or introduced, especially something relating to the law or politics
the institution of divorce proceedings
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.