One classic definition of organization development comes from Richard Beckhard’s 1969 Organization Development: Strategies and Models.
Organization Development is an effort (1) planned, (2) organization-wide, and (3) managed from the top, to (4) increase organization effectiveness and health through (5) planned interventions in the organizations “processes,” using behavioral-science knowledge.
Other definitions from leaders in the Organization Development Network:
OD is a field directed at interventions in the processes of human systems (formal and informal groups, organizations, communities, and societies) in order to increase their effectiveness and health using a variety of disciplines, principally applied behavioral sciences. OD requires practitioners to be conscious about the values guiding their practice and focuses on achieving its results through people.
-- Arnold Minors, Arnold Minors & Associates, Toronto, Canada
Organization Development is a body of knowledge and practice that enhances organizational performance and individual development, viewing the organization as a complex system of systems that exist within a larger system, each of which has its own attributes and degrees of alignment. OD interventions in these systems are inclusive methodologies and approaches to strategic planning, organization design, leadership development, change management, performance management, coaching, diversity, and work/life balance.
-- Matt Minahan, MM & Associates, Silver Spring, Maryland