dementia  praecox refers to a Latin term which means  "premature loss  of mind" is an early label for what is now called Schizophrenia,  emphasizing the disorder’s frequent appearance during adolescence.  
Other /More definition:
Dementia praecox refers to the term coined by Kraepelin to describe what is currently known as Schizophrenia. According to Kraepelin, this condition involves a degeneration of the brain that begins at a young age and ultimately leads to a disintegration of the entire personality.
It is a Latin term meaning "premature loss of mind"; an early label for what is now called Schizophrenia, emphasizing the disorder’s frequent appearance during adolescence.
    
        
        
                                        