The Bronx is New York City's northernmost borough, coterminous with Bronx County. The Bronx is located Northeast of Manhattan. It is the only one of the city's five boroughs situated primarily on the United States mainland rather than on an island. As of 2005, the United States Census Bureau estimated that the borough's population was 1,357,589. If all five boroughs were independent cities, the Bronx would rank as the ninth most populous city in the United States. Recently, its population, which had been in decline since the 1960 census, has increased. The borough had its peak population in 1950. The Bronx is the fourth most populous of New York City's five boroughs, and Bronx County is the 5th most populous county in the New York Metropolitan Area.
"The Bronx" is the only borough in New York City that, in its capacity as a borough, is referred to, in both law and popular usage, with the definite article ("The"). (The name of the coterminous "Bronx County," however, does not include a "the.") A common explanation for the definite article is that the original name of the borough, when it was annexed from Westchester, was "The Borough of the Bronx River," referring to the river that passed through the borough. The river was named after Jonas Bronck, a Swede, who was a sea captain and 1641 resident whose 500 acre (2 km²) farm lay between the Harlem River and the Bronx River or Aquahung, as it was called by the Indians of the time. Another explanation for the use of the definite article in the borough's name is that the original form of the name was possessive: The Broncks'.