Scheduling Fundamentals
Critical Path
The critical path is the sequence of connected activities in a schedule that determines the project's earliest possible finish date -- any delay to an activity on it delays the whole project by the same amount.
Definition
In Critical Path Method (CPM) scheduling, the critical path is the longest continuous chain of dependent activities from project start to project finish. Every activity on it has zero total float (or is otherwise flagged as 'driving' in the scheduling software), meaning it cannot slip without pushing the overall completion date. Activities not on the critical path have float -- some amount of slack before they, too, would start driving the finish date. Because the critical path can shift between schedule updates as logic, durations, and progress change, identifying exactly which activities were driving the finish date at any given point -- and whether that changed -- is central to any delay analysis.
Example
Comparing two schedule updates shows that 'Install Curtain Wall' left the critical path while 'Obtain Occupancy Permit' entered it -- direct evidence that the actual driver of the project's finish date changed between the two updates, not just that some float moved.