A milestone is one of the series of markers placed along road at intervals of one km or occasionally parts of it. Milestones are constructed to provide reference points along the road. This can be used to reassure travelers that the proper path is being followed and to indicate either distance travelled or the remaining distance to destination.
In project management context milestones are frequently used to monitor the progress. Milestones allow project manager to accurately determine whether or not project is on track.
Milestones usually show progress only on critical path and ignore non critical activities. It is common for resources to be moved from noncritical to critical activities to ensure milestones are met. This gives the impression that project is on schedule when actually some activities are being ignored.