The word nervous can indicted anything related to the nervous system. The Nervous system consists of grossly three parts, the Central Nervous System, the Peripheral Nervous system and the Somatic Nervous System. The neurons or the nerve cells forms the basic structural and functional unit of the nervous system. The nervous system not only control almost all the function of human body through brain, but also regulates the behaviour exhibited by an individual.
To look in a different way: In layman’s term, we use the word nervous to indicate people who usually turns anxious very frequently. Thus, psychologically speaking, a person who often become nervous, might be actually suffering from any anxiety disorder. the Diagnostic and Statistical Manual 5, by the American Psychological Association, have defined different types of anxiety disorders, including Generalized Anxiety Disorder, Phobias (Specific, Social or Agoraphobia), Panic Attack, or childhood anxieties like the Separation anxiety.