There are many twists on this definition, see http://www.cmcrossroads.com/bradapp/acme/scm-defs.html
The definition that I found to be the most practical and modern, is: "Software Configuration Management is the process of identifying, organizing, controlling, and tracking both the decomposition and recomposition of: software structure, functionality, evolution, and teamwork. In short, SCM is the "glue" between software artifacts, features, changes, and team members; it forms the ties that bind them all together from concept to delivery and beyond."
A shorter version of this: Controlling source code and development artifacts is a critical part of modern software development. This process is generally known as software configuration management."