The Gilded Age is the term applied to the period after the Civil War until about 1896. It was popularized by Mark Twain in the book "The Gilded Age: A Tale of Today," which he co-wrote with his friends Charles Dudley Warner and published in 1873. A "golden age" would be in contrast to one that is just gilded, i.e. with an overlay of gold on some baser metal.
While the term can be used for the historical period as a whole, it is more often used in reference to the ostentatious lifestyles of the very rich, along with their cutthroat business philosophies. It was a period when many Americans gained a degree of prosperity through the advances of industry, transportation, and commerce, but the concentration of wealth in the hands of a few, and their concentration in a few of the largest cities, created a visible contrast. The Vanderbilts built their mansions on Fifth Avenue and in Newport, hosting lavish parties, while the poor were crammed into the Tenements of the Lower East Side only a few blocks removed.
It had been a tenet of American national identity that extremes of wealth were features of the decadent Europeans. During the Gilded Age, it became for a period of time socially and intellectually acceptable. Ostentation began to fall out of fashion during the Progressive Era and after a brief resurgence during the Roaring Twenties disappeared for good during The Great Depression. Great wealth still exists, but public displays are no longer in fashion.