Critical path
Beginner
The slowest route a signal takes through the circuit. It sets how fast the whole chip can run.
Novice
The timing path with the least slack, usually from one flip-flop through a long chain of gates to the next. Its delay sets the fastest clock the design can use.
Expert
The path with the worst slack in a path group. Optimization spends most of its effort there and recovers area everywhere else. Fixing one usually exposes the next-worst, so look at how many paths fail and by how much, not only at the worst one.