The term critical activity refers to any particular schedule activity that happens to be part of a critical path that takes place within a project schedule. Critical activities are most commonly determined through the implementation and utilization of the of the critical path method (described as part of a separate entry). It is important to consider the usage of the term “critical ” in regards to project management functionality is rarely used in the context that one would find in a standard dictionary. When described typically and in common usage, the term critical can refer to something that is essential, something that is vital, or something of extreme importance. However, the term critical rarely means this in the context of project management. In project management terms, critical (such as critical activity) refers to the primary, such as being on the primary (or critical path), the main path of the life of an activity.
This term is defined in the 3rd and the 4th edition of the PMBOK.