This refers to the closing of the frontier. In 1890, the US Census Bureau declared that the frontier had closed in the sense that there was no longer any free land for people to take. The declaration would have probably gone largely unnoticed except that American historian Frederick Jackson Turner gave an important speech in 1893 talking about the closing of the frontier and its ramifications for American society and its democracy. Turners speech initiated a century-long debate on whether his observations were accurate.