presenile dementia (pri??si?na?l) n (Pathology) a form of dementia, of unknown cause, starting before a person is oldCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014