English Proper noun Klinefelter (plural Klinefelters) A surname. Derived terms Klinefelter syndrome, Klinefelter's syndrome Noun Klinefelter (plural Klinefelters) A person with Klinefelter syndrome. Statistics According to the 2010 United States Census, Klinefelter is the 20758th most common surname in the United States, belonging to 1273 individuals. Klinefelter is most common among White (93.72%) individuals. Wikiwand - on Seamless Wikipedia browsing. On steroids.