The term critical path refers to an input/output mechanism that is of fundamental and critical importance to the life of prosperity of a particular project. The critical path specifically refers to the sequence or order of schedule activities that will make up the entire duration of a project. Typically, the critical path represents the longest path through the entirety of the project and typically this path consists of every single activity that must be concluded from the kickoff to the ultimate conclusion of the project. However, in some cases a critical path can terminate on a particular schedule milestone that does not come at the conclusion of the project. However, in these cases, typically, the schedule milestone in question will have a finish that comes at a time no later than that of the imposed date schedule restraint in question. That said, in more cases than not the critical path does indeed run the entire life of a project. For more information, please see critical path method.
This term is defined in the 3rd and the 4th edition of the PMBOK.