According to PMBOK, Progressive Elaboration is a characteristic of projects that integrates the concepts of temporary and unique. Because the product of each project is unique, the characteristics that distinguish the product or service must be progressively elaborated.
Progressively means “proceeding in steps; continuing steadily by increments, ”while elaborated means “worked out with care and detail; developed thoroughly”. These distinguishing characteristics will be broadly defined early in the project, and will be made more explicit and detailed as the project team develops a better and more complete understanding of the product.
Progressive elaboration of product characteristics must be carefully coordinated with proper project scope definition, particularly if the project is performed under contract. When properly defined, the scope of the project — the work to be done — should remain constant even as the product characteristics are progressively elaborated.