  The author is very grateful for comments on previous drafts of this entry to Ralf Bader, Katherine Brading, Jacob Busch, Anjan Chakravartty, Michael Esfeld, Katherine Hawley, Steven French, Øystein Linnebo, and Stathis Psillos. The editors would like to thank Christopher von Bülow and Gintautas Miliauskas for carefully reading this entry and identifying numerous typographical errors.