Why is the time period between 1865 thru 1900 called Gilded age?

The "Gilded Age" term came into use in the 1920s and 1930s and was derived from writer Mark Twain's and Charles Dudley Warner's 1873 novel The Gilded Age: A Tale of Today, which satirized an era of serious social problems masked by a thin gold gilding.

Explanation:

Mark Twain called the late 19th century the "Gilded Age." By this, he meant that the period was glittering on the surface but corrupt underneath.