The concept of progressive elaboration refers specifically to a project management technique in which the plan for the particular and designated project is being continuously and constantly modified, detailed, and improved as newer and more improved (as well as more highly detailed), sets of information becomes available to the project management team and the project management team leader as the project unfolds and begins taking place. As a result f this new influx of information, the newly revised project management plan has the distinct quality of being more accurately drawn up and, in the end, more complete. These newly formed plans, which were derived ultimately from a series of successive iterations as more and better information has been made available to the project management team. Progressive elaboration is a fundamentally important step to the project management planning process as it can take a sketchy preliminary plan and refine it.
This term is defined in the 3rd and the 4th edition of the PMBOK.