A penitentiary is a large prison where people serve long sentences, usually for serious crimes. The word comes from an old idea that prisoners should feel penitent, meaning genuinely sorry for what they've done, and use their time locked away to think about their actions and become better people.

In the United States, penitentiaries are typically run by state or federal governments and house people convicted of major felonies rather than minor offenses. A person might be sentenced to ten years in a penitentiary for crimes like robbery or fraud. The thick walls, guard towers, and high fences of a penitentiary are designed to keep people from escaping while they serve their time.

The word penitentiary sounds more formal than prison or jail, and it's often used in official contexts. You might read that someone was “sentenced to five years in the state penitentiary.” Some famous penitentiaries, like Alcatraz in San Francisco Bay, have become tourist attractions after closing down. Others, like the federal penitentiary at Leavenworth, Kansas, remain active and well-known.

While penitentiary and prison mean essentially the same thing today, jail is different: jails hold people for shorter periods, often while they await trial or serve sentences of less than a year.