An institution is an organization or custom that has become an established and important part of society. Schools, hospitals, courts, and libraries are all institutions because they serve essential functions and have existed for a long time. The Supreme Court is an institution of American government. The Smithsonian is an institution dedicated to education and research.

The word extends beyond buildings or organizations to describe established patterns in society. When something becomes institutionalized, it becomes a regular, expected part of how things work. Baseball is sometimes called an American institution because it's been woven into our culture for over a century. Family dinner might be an institution in your household: a regular practice that everyone expects and values.

The word can also refer to places that care for people who need special help or supervision, like mental health institutions or correctional institutions. These facilities provide structured environments for people who need them.

When you hear that someone is “an institution” themselves, it means they've become such a permanent, valued part of a place that imagining it without them seems impossible. A teacher who has worked at the same school for thirty years might be called an institution, someone whose presence helps define the place itself.