1890 Census: Birthplace
The individual Census Takers who would visit your home or farm were given a list of codes for each possible place you may have been born.
Keep in mind that the United States of 1890 was very much a country of immigrants. Many of those immigrants were coming (or had recently come) from Europe. The trip would take around 7 days1 , even by the fastest steamships in 1890, so the census list actually has two separate codes for those people born at sea.
1 Ocean Passenger Travel - The Speed of the 1890 Steamships, http://www.gjenvick.com/SteamshipArticles/TransatlanticShipsAndVoyages/OceanPassengerTravel/1891/04-SpeedOf1890Steamships.html