What was the Gilded Age, who named it, and why that name?

In American history, the Gilded Age refers to substantial growth in population in the United States and extravagant displays of wealth and excess of America's upper class during the post-Civil War and post-Reconstruction eras of the late 19th century (1865-1901). The term "Gilded Age" was coined by Mark Twain and Charles Dudley Warner in their 1873 book, The Gilded Age: A Tale of Today.

https://en.wikipedia.org/wiki/Gilded_age

It appears fairly self-explanatory -- However -- Mark Twain called the late nineteenth century the "Gilded Age." By this, he meant that the period was glittering on the surface but corrupt underneath.

http://www.digitalhistory.uh.edu/era.cfm?eraID=9&smtid=1