OD (Organizational Development) is a process that applies behavioral science knowledge to help organizations achieve greater effectiveness and improved quality of work life. It involves assessing an organization's current functioning and improving the total system from a large perspective. OD is defined as a system wide application of behavioral science knowledge to strategically develop and reinforce organizational structures, processes, and changes aimed at improving overall effectiveness. The document then provides explanations of key terms in the definition and discusses reasons for OD along with its historical background.