n
[C]
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
[C]
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.
[C]
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)
[U]
when something is started or introduced, especially something relating to the law or politics
institution of
　the institution of divorce proceedings
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.