Fluctuations of the price of any company's share than the average change of total share-value of the market is called Beta-co-efficient.
According to Besley and Brigham,” Sensitivity to market fluctuations is called its beta coefficient.”
According to Khan and Jain,” The beta-coefficient its an index of the degree of responsiveness/co-movement of security return with market return.”
According to L.J. Gitman,” The beta coefficient measures non diversifiable risk. It is an index of the degree of movement of an assets return in response to a change in the market return.”