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Hillside, New Jersey. Hillside is a township in Union County, New Jersey, United States. As of the 2010 United States Census, the township's population was 21,404, reflecting a decline of 343 (-1.6%) from the 21,747 counted in the 2000 Census, which had in turn increased by 703 (+3.3%) from the 21,044 counted in the 1990 Census.
Hillside is located in the 10th Congressional District and is part of New Jersey's 20th state legislative district. Prior to the 2011 reapportionment following the 2010 Census, Hillside had been in the 29th state legislative district.