A useful definition of change management that I use is:
‚the coordination of a structured period of transition from situation A to situation B in order to achieve lasting change within an organization‘.
(BNET Business Dictionary)
To help you in your search for a definition of change management here are others I’ve found to be useful:
The systematic approach and application of knowledge, tools and resources to deal with change. Change management means defining and adopting corporate strategies, structures, procedures and technologies to deal with changes in external conditions and the business environment.
SHRM Glossary of Human Resources Terms, http://www.shrm.org.
Change management is the process, tools and techniques to manage the people-side of business change to achieve the required business outcome, and to realize that business change effectively within the social infrastructure of the workplace.
Change Management Learning Center
Change Management: activities involved in (1) defining and instilling new values, attitudes, norms, and behaviors within an organization that support new ways of doing work and overcome resistance to change; (2) building consensus among customers and stakeholders on specific changes designed to better meet their needs; and (3) planning, testing, and implementing all aspects of the transition from one organizational structure or business process to another.
http://www.gao.gov/special.pubs/bprag/bprgloss.htm
…a systematic approach to dealing with change, both from the perspective of an organization and on the individual level…proactively addressing adapting to change, controlling change, and effecting change.
Case Western Reserve University
Change management is a systematic approach to dealing with change, both from the perspective of an organization and on the individual level.
searchsmb.com
Change Management is an organized, systematic application of the knowledge, tools, and resources of change that provides organizations with a key process to achieve their business strategy.
Lamarsh
The systematic management of a new business model integration into an organization and the ability to adapt this change into the organization so that the transformation enhances the organizational relationships with all its constituents.
bitpipe.com
Change Management: the process, tools and techniques to manage the people-side of change processes, to achieve the required outcomes, and to realize the change effectively within individuals, teams, and the wider systems.
Change management is a structured approach to transitioning individuals, teams, and organizations from a current state to a desired future state. The current definition of Change Management includes both organizational change management processes and individual change management models, which together are used to manage the people side of change.
Wikipedia
Minimizing resistance to organizational change through involvement of key players and stakeholders.
BusinessDictionary.com
Change management is a style of management that aims to encourage organizations and individuals to deal effectively with the changes taking place in their work.
English Collins Dictionary
quote: http://www.change-management-coach.com/definition-of-change-management.html
TheIdea – The Innovation Agency 
The IOCMI Definition of Organizational Change management is „The planning, Leading, Organizing and Controlling of Strategic Change.“