Lehigh County (Pennsylvania German: "Lechaa Kaundi") is a county located in the Lehigh Valley region of the U.S. state of Pennsylvania. As of the 2010 census, the population was 349,497. Its county seat is Allentown, the state's third-largest city behind Philadelphia and Pittsburgh. The county, which was first settled around 1730, was formed in 1812 with the division of Northampton County into two counties. It is named after the Lehigh River, whose name is derived from the Delaware Indian term Lechauweki or Lechauwekink, meaning "where there are forks".