Definition:
Rolling Wave Planning is the process of project planning in waves as the project proceeds and later details become clearer. Work to be done in the near term is based on high level assumptions; also, high level milestones are set. As the project progresses, the risks, assumptions, and milestones originally identified become more defined and reliable. One would use Rolling Wave Planning in an instance where there is an extremely tight schedule or timeline to adhere to; whereas more thorough planning would have placed the schedule into an unacceptable negative schedule variance. This is an approach that iteratively plans for a project as it unfolds, similar to the techniques used in Scrum (development) and other forms of Agile software development. The concepts of Rolling Wave Planning and Progressive Elaboration are techniques within the PMBOK Define Activities process and it also describes how the Project Management Plan evolves.
Further Reading:
Book: Playing to Win by A G Lafley and Rogel L. Martin