The term decomposition, used in non project-related terms to describe an item that has deteriorated in quality, or broken down into more elemental components. And this definition is actually not terribly deviant from the definition of this term in the arena of project management. In project management terms, the term decomposition refers to a specific type of planning technique that takes the entire project scope and all project deliverables and breaks them down into smaller components. These smaller components are typically significantly easier to manager, and this new, broken down assemblage of components is typically maintained until the project work that can be associated with actual fulfillment of the project scope and provision of all project deliverables has been sufficiently defined in such that all efforts in properly controlling, monitoring, and executing the work are in a state in which they can be readily addressed and completed by the project management team.’
This term is defined in the 3rd and the 4th edition of the PMBOK.