Autonomic Nervous System:

The part of the nervous system that is responsible for control and regulation of the involuntary bodily functions, including those of the heart, blood vessels, visceral smooth muscles and glands: it consists of the sympathetic system which, in general, stimulates the body to prepare for physical action or emergency, and the parasympathetic system which, in general, stimulates the opposite responses.*

Parasympathetic Nervous System:

That part of the autonomic nervous system whose nerves originate in the midbrain, the hindbrain, and the sacral region of the spinal cord and whose functions include the constriction of the pupils of the eyes, the slowing of the heartbeat, and the stimulation of certain digestive glands: these nerves oppose the sympathetic nerves in the regulation of many body processes.

Sympathetic Nervous System:

That part of the autonomic nervous system whose nerves originate in the lumbar and thoracic regions of the spinal cord and that is especially concerned with mediating the involuntary response to alarm, as by speeding the heart rate, raising the blood pressure and dilating the pupils of the eyes: these nerves oppose the parasympathetic nerves in the regulation of many body processes.

* From Webster’s 3rd College Edition