What does the "R" stand for on the rating of a movie?

On the U.S. movie rating system, the “R” stands for Restricted. An R-rated film contains material that may be unsuitable for children, and anyone under 17 generally must attend with a parent or adult guardian.

The rating is issued through the Classification and Ratings Administration, an independent division of the Motion Picture Association. The modern U.S. system began in 1968, replacing Hollywood's older Production Code, which had governed acceptable screen content more directly.

R does not mean that nobody under 17 can see the movie. That restriction applies more strictly to NC-17, which means no one 17 or under is admitted. R-rated films may receive the classification for strong language, violence, sexual content, drug use, or a combination of factors; the rating often includes a brief explanation of the specific content.

A further point of confusion is that movie classifications vary by country. “R” can have a different age threshold or meaning in systems such as Australia's or New Zealand's. In this question, the intended reference is the American MPA rating system.