subacute sclerosing leukoencephalitis n 1: a rare chronic progressive encephalitis caused by the measles virus and occurring primarily in children and young adults; death usually occurs within three years; characterized by primary measles infection before the age of two years [syn: {subacute sclerosing panencephalitis}, {SSPE}, {inclusion body encephalitis}, {subacute inclusion body encephalitis}, {sclerosing leukoencephalitis}, {subacute sclerosing leukoencephalitis}, {Bosin's disease}, {Dawson's encephalitis}, {Van Bogaert encephalitis}]