The Mount Rushmore of Change Management
Though often attributed to Dean and Linda Anderson (because the definition appears in the forward to their book), the most authoritative definition of change management was crafted by Daryl Conner, the author of Managing at the Speed of Change. Here it is:
Change management is a set of principles, techniques, and prescriptions applied to the human aspects of executing major change initiatives in organizational settings.
Its focus in not on "what" is driving change (technology, reorganization plans, mergers/acquisitions, globalization, etc.), but on "how" to orchestrate the human infrastructure that surrounds key projects to that people are better prepared to absorb the implications affecting them.
Tim Creasey from the Change Management Learning Center (developers of ADKAR) provides a short, sweet, and to the point definition:
the process, tools and techniques to manage the people-side of change to achieve a required business outcome.