A nudist is a person who likes to go without clothes because they believe it is natural and healthy. The word also works as an adjective: a nudist beach, a nudist camp — a place set aside for that. It is more everyday than nudism, because we talk about people and places more than about -isms. Some people in the same group prefer naturist, which can sound a little softer. In jokes and films the word can be made to sound naughty; among the people themselves it is just a name for their choice. You do not call a baby in the bath a nudist, and you do not call an art model a nudist unless they belong to that movement.