1 definition found islip - U.S. Gazetteer (1990) : Islip, NY (CDP, FIPS 37869) Location: 40.73277 N, 73.21765 W Population (1990): 18924 (6458 housing units) Area: 14.0 sq km (land), 0.9 sq km (water) Zip code(s): 11751