1. A building into which people are legally committed as a punishment for a crime or while awaiting trial: A large prison is called a "penitentiary" and the people who are kept there are called "prisoners".
2. Etymology: just like prehensile, comprehend, comprehensible, etc., prison essentially comes from Latin prehendere, "to seize".

From this origin, the noun prehensio, "seizure", was contracted to prensio, which in Old French was presented as prisun.

At that time, it came to be used specifically for "imprisonment" and then it became the "place of imprisonment" or a "prison".