The Gilded Age in United States history is the late 19th century, from the 1870s to about 1900. The term for this period 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. The early half of the Gilded Age roughly coincided with the middle portion of the Victorian era in Britain and the Belle Epoque in France. Its beginning in the years after the American Civil War overlaps the Reconstruction Era (which ended in 1877). It was followed in the 1890s by the Progressive Era.

Based on this passage, to what period of time does the term gilded age refer? Let's give stream of consciousness first:
The relevant sentence in the passage is: The Gilded Age in United States history is the late 19th century, from the 1870s to about 1900.
So, the final answer is the late 19th century , from the 1870s to about 1900.