V-Chip

Definition
The Federal Communications Commission (FCC) requires all television sets with picture screens 33 centimeters (13 inches) or larger to be equipped with features to block the display of television programming based upon its rating. This technology is known as the V-Chip. The V-Chip reads information encoded in the rated program and blocks programs from the set based upon the rating selected by the parent.

The rating system, also known as the "TV Parental Guidelines," was established by the National Association of Broadcasters, the National Cable Television Association and the Motion Picture Association of America. These ratings are displayed on the television screen for the first 15 seconds of rated programming and, in conjunction with the V-Chip, permit parents to block programming with a certain rating from coming into their home.

Source
Federal Communications Commission, "V-Chip" (full-text).