Throughout the entire life cycle of a project, there are going to be a number of natural time gradients that exists, and are more than likely to be defined by the eventual and natural ebbing and flowing of the project workload and the momentum of the team’s performance and the business of the schedule. However, it is typically helpful in attempting to assure that the project is moving effectively as well as to allow points in time for the project team to pause and look back on what has been accomplished to date as well as what may need to be changed in the future for the project team leader with or without the help of the project team to establish a number or series of project milestones. These milestones can occur at any point throughout the project and specifically refer to any significant or substantive point, time, or event in the life cycle of the project. These milestones typically refer to points at which large schedule events or series of events have been completed, and a new phase ort phases are set to begin. See also schedule milestone.
This term is defined in the 3rd and the 4th edition of the PMBOK.