Activity definition refers to the process of parsing a project into a number of individual tasks which must be completed before the deliverables can be considered completed. Activity definitions rely on a number of specific input processes. These include enterprise environmental factors, organizational process assets, the project scope statement, the work breakdown structure, the WBS dictionary, the project management plan (which consists of the project scope management plan and the schedule management plan). Through use of these inputs, the activities that will need to ultimately be completed in order to compete all of the project objectives can be readily defined, and the activity definition can be further accomplished through the utilization of a number of tools and techniques including decomposition, templates, rolling wave planning, expert judgment, and planning components. The four ultimate outputs of the activity definition process are the activity list, the resulting activity attributes, all requested changes, and any milestones.

This term is defined in the 3rd edition of the PMBOK but not in the 4th.