1. A sympathetic nerve or the sympathetic nervous system is a set or series of interconnected or interdependent parts of entities, objects, organs, or organisms that function together in a common purpose or produce results impossible of achievement by one of them acting or operating alone.
2. A part of the nervous system which serves to accelerate the heart rate, to constrict blood vessels, and to raise blood pressure.

The sympathetic nervous system and the parasympathetic nervous system are parts of the autonomic nervous system.