"Description and discussion of the many aspects of Parkinson disease (PD) is dogged by the fact that PD is an ambiguous concept. What we call PD is an etiologically and clinically heterogeneous meta-syndrome overlapping a number of other disorders. Definitions, diagnostic criteria, and classification schema provide essential common vocabularies for communication among clinicians and researchers. Definitions, diagnostic criteria, and classification schemes for PD, however, are constructs imposed on variable and imperfectly understood disease biology. Difficulties with formulation of PD definitions, diagnostic criteria, and classification schemes include historical and contemporary use of competing definitions of PD, disease heterogeneity, the existence of disorders with overlapping clinical and pathological features, and ambiguous use of the terms disease and syndrome. For the purposes of exposition and discussion in this book, an inclusive, perhaps vague, definition of PD is employed:"--