What is the name of the Disneyland land themed to the American Old West?

The Disneyland land themed to the American Old West is Frontierland.

Frontierland was one of Disneyland's five original themed lands when the park opened on July 17, 1955. Walt Disney envisioned it as a romanticized version of 19th-century westward expansion, filled with cowboys, pioneers, saloons, mining imagery and wide-open wilderness.

Early experiences included stagecoach rides, pack-mule excursions, Conestoga wagons and walking trails. The land later gained larger attractions, including Mine Train Through Nature's Wonderland in 1960. That attraction closed in 1977, and Big Thunder Mountain Railroad opened in its place in 1979.

Westernland is a related name used for the equivalent land at Tokyo Disneyland, but it is not Disneyland's official name. Critter Country and Adventureland have different themes: one centers on woodland characters and the other on exploration. Frontierland's Rivers of America shoreline also provides a major setting for nighttime entertainment.