In medicine, Parkinson's disease (PD), also called primary Parkinsonism, Paralysis Agitans, and other names, is a disease affecting movement. "The four primary symptoms of PD are tremor, or trembling in hands, arms, legs, jaw, and face; rigidity, or stiffness of the limbs and trunk; bradykinesia, or slowness of movement; and postural instability, or impaired balance and coordination". In other words, Parkinson's disease is a Parkinsonian disorders, but not all Parkinsonian disorders are true Parkinson's disease.
Formally, it is progressive, degenerative neurologic disease characterized by a tremor that is maximal at rest, retropulsion (i.e. a tendency to fall backwards), rigidity, stooped posture, slowness of voluntary movements, and a masklike facial expression. Pathologic features include loss of melanin containing neurons in the substantia nigra and other pigmented nuclei of the brainstem. Lewy bodies are present in the substantia nigra and locus coeruleus but may also be found the related diffuse Lewy body disease characterized by dementia in combination with varying degrees of parkinsonism. (Adams et al., Principles of Neurology, 6th ed, p1059, pp1067-75) More specifically, 
Common to the types is the death or inability of cells in the substantia nigra to produce the neurotransmitter, dopamine; dopamine replacement and modulation is at the center of medical treatment. Some forms of Parkinson disease are inherited via genes such as leucine-rich repeat kinase 2 (LRRK2).
- NINDS Parkinson's Disease Information Page, National Institute of Neurological Diseases and Stroke
- Anonymous (2015), Parkinson's disease (English). Medical Subject Headings. U.S. National Library of Medicine.
- Online Mendelian Inheritance in Man, OMIM®. Johns Hopkins University, Baltimore, MD. MIM Number: 168600. World Wide Web URL: http://omim.org/.
- Online Mendelian Inheritance in Man, OMIM®. Johns Hopkins University, Baltimore, MD. MIM Number: 609007. World Wide Web URL: http://omim.org/.