Nomenclature
Initially it was known as the Independent City-Owned Subway System (ICOS), Independent Subway System (ISS), or Independent City-Owned Rapid Transit Railroad. It became known as the IND after unification of the subway lines in 1940; the name IND was assigned to match the three-letter initialisms of the IRT and BMT.
The first IND line was the Eighth Avenue Line in Manhattan, opened on September 10, 1932; for a while the whole system was colloquially known as the Eighth Avenue Subway. The original IND system was entirely underground in the four boroughs that it served, with the exception of a short section of the IND Culver Line containing two stations spanning the Gowanus Canal in the Gowanus section of Brooklyn.
Read more about this topic: Independent Subway System