Basically the cough is an action of the body to get rid of any substance that irritates the air passage. When certain special cells along with the air passage get irritated and trigger a chain of events, they are labeled as cough or coughing.
This is defined as expel of an air from the lungs suddenly and noisily. This is often done to keep the respiratory tract free from the irritating materials. Ayurveda believes this to be kaasa meaning the sound that is produced when you crash two utensils or stones with each other.
The patient seeks medical treatment of cough more than that of any other complaints. The treatment is costing one billion dollars annually only in USA.