Definition of "Institution" :

noun: the act of starting something for the first time; introducing something new

noun: an establishment consisting of a building or complex of buildings where an organization for the promotion of some cause is situated

noun: a hospital for mentally incompetent or unbalanced person

noun: a custom that for a long time has been an important feature of some group or society

"The institution of marriage."

noun: an organization founded and united for a specific purpose