It wasn't until Frederick Jackson Turner published The Significance of the Frontier in American History in 1893 that Americans fully realized there had been a frontier. In what was probably the most significant and controversial piece of scholarship of the 1890s, Turner showed that the nation's identity in the 19th century had hinged on westward expansion; now a new identity would be required.

"Open-source software" is another one of those concepts that gives historical unity to decades of bewildering change. The term didn't enter circulation until early 1998, but in the two years since, the revolutionary idea behind it has caught hold in boardrooms and IT departments across Silicon Valley and beyond.