institucio

Institucio and instituto are two words, not one. Instituto is a named establishment with a building and a payroll: a research institute, a language institute, the sort of thing that appears on letterhead. Institucio is broader and more abstract: an established body, arrangement or practice in society. Marriage is an institucio. So is the monarchy, so is trial by jury, and so is the village pub if you want to be affectionate about it. A university is both, depending on which aspect you mean.

The Latin says it plainly. Both words come from instituere, to set up, built on stare, to stand. What survives in the Esperanto pair is the difference between a thing somebody set up (instituto) and the fact of a thing standing (institucio).

Around it. Fondi is to found, fondaĵo is a foundation as an endowed body, societo is a society or association, and organizaĵo is an organisation. Institucia is institutional; instituciigi is to institutionalise a practice, and it carries the same faintly bureaucratic chill it does in English.