A monastery is the building where monks live while they’re practicing their religion. Some monasteries are occupied by hundreds of monks, and sometimes only one monk lives there all alone.
Christians have a church, Jews have a synagogue, and monks have a monastery, which serves not only as their holy place of worship, but also where they eat and sleep. You’ll use the word mostly when referring to monks, but there are some monasteries for Catholic nuns. The Greek word monastērion means “a place to live alone,” and monks and nuns go to a monastery to focus on their religion in isolation, away from the rest of the world.