 The first version of this entry was written by Antony Galton in 1999, later revised in Galton (2008). In 2015 the entry was substantially re-written and extended by the first author of the present version, which is itself a major revision and further extension of the 2015 version. We acknowledge Galton’s contribution to the previous versions, and we retained one example from Galton (2008) in Section 8.1, as indicated there. We are grateful to Johan van Benthem, Rob Goldblatt, Angelo Montanari, Yde Venema, Michael Zakharyaschev, Ed Zalta, and Alberto Zanardo for some helpful suggestions and corrections on the 2015 version.